Fixes https://github.com/flutter/flutter/issues/18852

Creates an initial, minimal FLEEngine that parallels the iOS
FlutterEngine, and moves engine launch, plugin registration, and binary
messenger functionality from FLEViewController to there. Uses the FLE
prefix since the current APIs will change to better match FlutterEngine;
the current APIs are intended to minimize conceptual changes relative to
the current FLEViewController APIs since it's already a substantial
change.

Further changes will move channels from FLEViewController to FLEEngine,
and further align the APIs and functionality with the iOS version.

Part of #32421

* Make pipeline hold a deque so we can push_front

- Also assert that all the DoDraw tasks are executed
  on the GPU thread.

* Add tests for pipeline

* Add support for pipeline to push resources to the front

Made the engine and the view controllers have BinaryMessengers, not be
BinaryMessengers.  This allows us to break retain cycles and makes the
leaking channels we have not less dire.

* Revert "Revert "IOS Platform view transform/clipping (#9075)" (#9480)"

This reverts commit 00d929f7.

* fix fuschia buid

* Remove deprecated runBundle APIs
* Remove code related to dynamic patching (including support for multiple
  bundle paths)
* Change FlutterRunArugments.bundlePath to be the Android AssetManager path
  where the app's assets are located

This reverts commit ebb5b909.

Seeing the following breakage on host build:
```
../../flutter/flow/scene_update_context.cc:205:36: error: non-const lvalue reference to type 'flutter::MutatorsStack' cannot bind to a value of unrelated type 'const flutter::Stopwatch'
                                   frame.context().raster_time(),
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../flutter/flow/scene_update_context.cc:207:36: error: no viable conversion from 'flutter::TextureRegistry' to 'const flutter::Stopwatch'
                                   frame.context().texture_registry(),
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../flutter/flow/instrumentation.h:55:32: note: candidate constructor not viable: no known conversion from 'flutter::TextureRegistry' to 'const flutter::Stopwatch &' for 1st argument
  FML_DISALLOW_COPY_AND_ASSIGN(Stopwatch);
                               ^
../../flutter/fml/macros.h:28:3: note: expanded from macro 'FML_DISALLOW_COPY_AND_ASSIGN'
  TypeName(const TypeName&) = delete;          \
  ^
../../flutter/flow/scene_update_context.cc:208:36: error: non-const lvalue reference to type 'flutter::TextureRegistry' cannot bind to a temporary of type 'flutter::RasterCache *'
                                   &frame.context().raster_cache(),
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../flutter/flow/scene_update_context.cc:209:36: error: cannot initialize a member subobject of type 'const flutter::RasterCache *' with an rvalue of type 'bool'
                                   false};
                                   ^~~~~
```

* Added unit tests for the ios code.

* Moved the tests to live next to the source.

* Added mocking library.

* Fixed formatting and removed third_party from the format check.

* fixed formatting 2

* Removed ocmock from third_party.

* Added ocmock to third_party, compile from source.

* removed ocmock from license checking

* updated licenses_flutter

* updated tool_signature

This reverts commit 2f36285b.

This was crashing the integration test `- (void)testDualFlutterView`.

Created FlutterSurrogateBinaryMessenger to make sure that channels are
holding onto engines and not viewcontrollers.  This doesn't change the
public API but makes clients do what we want them to be doing, using
Engine for FlutterBinaryMessenger.

When the macOS framework was initially landed, it included some iOS code
using a localized hack. This reorganizes the code structure to clearly
indicate which files are shared between iOS and macOS, vs. those that
are platform-specific.

* Refactor to move Task Queue to its own class

- This is to help with sharing task queue among
  multiple message loops going forward.

- currently there is 1:1 mapping between task queue
  and message loop, we are still maintaining the semantics
  for this change.

* Add mutex include

* Most of the waking up changes minus test failures

* Refactor MessageLoopImpl to be Wakeable

- Makes testing easier by letting us putting a TestWakeable

- Also move the waking up logic to the task queue

* add tests

* Fix formatting and license

SCN-947 #comment
SCN-1291 #comment

* Refactor to move Task Queue to its own class

- This is to help with sharing task queue among
  multiple message loops going forward.

- currently there is 1:1 mapping between task queue
  and message loop, we are still maintaining the semantics
  for this change.

* Add mutex include

* Add unit tests for task queue

* fix formatting

* license

* Refactor Delayed Tasks to their own class

* fix some comments

* Update BUILD.gn

This reverts commit 651c9040.

With this change applied, test/widgets/widget_inspector_test.dart is failing
when run by the dev/bots/test.dart script in the framework tree.

As another step toward aligning the macOS plugin API with the iOS plugin
API, and with its final form, rename the plugin-related classes from
FLEPlugin* to FlutterPlugin*. This makes the names of the clasess the
same between iOS and macOS. Eventually they should actually merge, with
annotations/ifdefs for platform differences, but for now since the macOS
version uses the same method names and is simply a subset of the iOS
APIs (with one to-be-stabilized method that is macOS-specific) doing the
merge later won't be a breaking change.

Doing this now allows for building out plugin tooling and plugin
implementations on macOS without creating a growing body of code that
will experience a breaking change later.

See:
https://github.com/flutter/flutter/issues/31735
https://github.com/flutter/flutter/issues/32718

BSDiff is no longer used as of flutter/engine#8663. This removes the
unused Java import and sources.

This improves performance as the version in the SDK is able to work more
incrementally and avoids creating an expensive ClassHierarchy.
There is now no measureable performance difference with the option so the
kernel transformer can be enabled by default.

Based on the iOS Flutter.podspec. This will be used for plugin
management via CocoaPods, as on iOS.

Part of https://github.com/flutter/flutter/issues/32718

Fixes https://github.com/flutter/flutter/issues/31691

New Plugin API PR3: Introduces Service, BroadcastReceiver, and ContentProvider awareness, control surfaces, and plugin bindings. (#8962)

This is forward compatible with std::size and similar to how Chromium
removed use of the arraysize macro.

New Plugin API PR2: Introduces ActivityAware, ActivityControlSurface, and ActivityPluginBinding. (#8943)

This will reduce executable size.

New Plugin API PR1: Introduces PluginRegistry and FlutterPlugin, adds support for plugin registration to FlutterEngine. (#8826)