Delegate a11y events and action to/from embedded Android platfrom views.

This handles delegation of:
  * AccessibilityNodeProvider#performAction
  * ViewGroup#requestSendAccessibilityEvent
  * View#onHoverEvent

Additionally updates the currently input accessibility focused node state that is
tracked by the a11y bridge when an embedded view's node is focused.

This PR mirrors virtual a11y tree of embedded platform views in the Flutter
a11y tree.

Non virtual hierarchies are not currently supported.

Only works on Android versions earlier than Android P as it relies on
reflection access to hidden system APIs which cannot be done starting
Android P.

A11y is not yet working as we also need to delegate a11y events from the
platform view to the FlutterView. This will be done in a following PR to
keep the change size a little saner.

Changes include:
- File structure
- Header guards
- Include paths
- Namespaces
- Integration with the engine's GN build
- Conversion from jsoncpp to rapidjson
- Style and clang-format adjustment to match engine repository

This is in preparation for implementing platform views a11y on Android.

And e2e working prototype is available here: https://github.com/amirh/engine/tree/a11y_hacks

flutter/flutter#19418

This reverts commit f1f19bba.

Now we shouldn't break the engine build as https://chromium-review.googlesource.com/c/chromium/tools/build/+/1480746 is landed. The golden test is disabled by default and we'll enable it later in our recipe and test it in presubmit tests.

There is already fml/memory/thread_checker.h

Android Embedding PR 9: Introduce an AndroidTouchProcessor to convert MotionEvents to Flutter touch data. (#7947)

Android Embedding PR 6: Introduce FlutterView structure with FlutterSurfaceView and FlutterTextureView. (#7896)

We should not be exporting anything from FML anyway.

Fixes https://github.com/flutter/flutter/issues/28240

Revert "Revert "Revert "Reland PerformanceOverlayLayer golden test (#7863)" (#7895)" (#7911)" (#7916)

This breaks the Linux build with:
```
[ RUN      ] PerformanceOverlayLayer.Gold
../../flutter/flow/layers/performance_overlay_layer_unittests.cc:70: Failure
Value of: golden_data != nullptr
  Actual: false
Expected: true
Golden file not found: "/b/s/w/ir/k/src/flutter/testing/resources"/performance_overlay_gold.png.
Please either set --golden-dir, or make sure that the unit test is run from the right directory (e.g., flutter/engine/src).
```

This reverts commit 3f0ce769.

This reverts commit 6d8bd99a.

Reverting this broke the flow tests for the performance overlay:

```
../../flutter/flow/layers/performance_overlay_layer_unittests.cc:70: Failure
Value of: golden_data != nullptr
  Actual: false
Expected: true
```

This reverts commit 8427d73c.

Flutter's accessibility APIs consist of three main calls from the
embedder to the Dart application:

  1. FlutterEngineUpdateSemanticsEnabled: enables/disables semantics support.

  2. FlutterEngineUpdateAccessibilityFeatures: sets embedder-specific
     accessibility features.

  3. FlutterEngineDispatchSemanticsAction: dispatches an action (tap,
     long-press, scroll, etc.) to a semantics node.

and two main callbacks triggered by Dart code:

  1. FlutterUpdateSemanticsNodeCallback: notifies the embedder of
     updates to the properties of a given semantics node.

  2. FlutterUpdateSemanticsCustomActionCallback: notifies the embedder
     of updates to custom semantics actions registered in Dart code.

In the Flutter framework, when accessibility is first enabled, the
embedder will receive a stream of update callbacks notifying the
embedder of the full semantics tree. On further changes in the Dart
application, only updates will be sent.

Fixes https://github.com/flutter/flutter/issues/28229

Merging back in after reversion. Fixed some messaging issues in FlutterNativeView and corrected some message parsing.

* Revert "Revert "Add mock capability to PerformanceOverlayLayer (#7537)" (#7765)"

This reverts commit 693645e4.

* Add command line args for golden dir and font file

This reverts commit f4fba6d7.

This reverts commit 0d6ff166.

This reverts commit d48de7a3.

From build log:
```
../../flutter/shell/platform/android/io/flutter/view/FlutterView.java:95: error: cannot find symbol
    private final LocalizationChannel localizationChannel;
                  ^
  symbol:   class LocalizationChannel
  location: class FlutterView
../../flutter/shell/platform/android/io/flutter/view/FlutterView.java:172: error: cannot find symbol
        localizationChannel = new LocalizationChannel(dartExecutor);
                                  ^
  symbol:   class LocalizationChannel
  location: class FlutterView
2 errors
```

This reverts commit 256db4bc.

The shell was already designed to cleanly shut down the VM but it couldnt
earlier as |Dart_Initialize| could never be called after a |Dart_Cleanup|. This
meant that shutting down an engine instance could not shut down the VM to save
memory because newly created engines in the process after that point couldn't
restart the VM. There can only be one VM running in a process at a time.

This patch separate the previous DartVM object into one that references a
running instance of the DartVM and a set of immutable dependencies that
components can reference even as the VM is shutting down.

Unit tests have been added to assert that non-overlapping engine launches use
difference VM instances.

This reverts commit 5f3f3bd9.

So we can do golden test on PerformanceOverlay to avoid regression like
https://github.com/flutter/flutter/issues/26387

Avoids potential confusion due to the iOS framework already being called
Flutter.framework.

Merges the current flutter-desktop-embedding macOS framework into the
engine. Notable changes:
- All channel/codec related code is eliminated in favor of using the
  existing iOS implementations.
- All .m files renamed to .mm for consistency with the iOS code.
  - Some minor code changes to fix new warnings in Objective-C++ mode.
- License headers, basic format (e.g., clang-format changes) updated to
  use repo style.
- Xcode project is not included; instead adds GN build rules to create
  an integrated framework that combines what was the FDE library with
  what is present in FlutterEmbedder.framework.

Other changes are left as follow-ups, including:
- Moving shared code out of ios/ into common/.
- Potentially improving sharing between iOS and macOS BUILD.gn.
- Class renaming; the FLE prefix will be eliminated, but that API
  surface isn't stable yet, so that can be changed later.

- Moved versions from shell to common
- versions singleton contains all the required versions.