This reverts commit 7932ba76.

The Android and iOS versions of this function had diverged. This patch
makes the iOS version match the Android version (and what the framework
expects).

Fixes https://github.com/flutter/flutter/issues/8878

Breaking change: removed facilities for JSON and string messaging from FlutterView/FlutterViewController, leaving only binary messaging there. All other use of flutter communication now goes through FlutterMessageChannel and FlutterMethodChannels. Retained use of String and JSON codecs for now.

Companion flutter PR: flutter/flutter#8837

Names matter, and I apparently forgot to test after adding ' Engine' to
the name. Names can't contain spaces, and must match the filename.

Flutter channels now also supported on iOS. Use FlutterMessageChannel or FlutterMethodChannel with binary/string/json/standard codecs.

* Add podspec to Flutter iOS framework.

Flutter iOS apps are already pod-enabled, so we might as well turn the
engine framework into a pod, so we don't have to manually copy the
current Flutter.framework into the app directory on every build, but
rather let Cocoapods wire things up for us.

For that to happen, we need a pod spec for Flutter.framework. This spec
will not be published, but rather downloaded as part of the engine
artifacts, and the app's Podfile will have a local path dependency on
it.

* Update licenses_golden...

This change eliminates two previous assumptions:
1. If a Flutter view is embedded in a parent view, that the parent view
   controller would apply any status bar padding necessary.
2. That we should not apply padding unless the flutter view is
   fullscreen.

A simple case where the first assumption fails to hold is a Flutter view
embedded in a UINavigationController with a hidden toolbar. A simple
case where the second assumption fails to hold is a view in a window
whose top overlaps the status bar but isn't fullscreen.

Ensure that the text input keyboard is reloaded if the view keyboardType
changes.

Added:
* UIRequiredDeviceCapabilities=[arm64]
* MinimumOSVersion=8.0

Provide a default status bar tap handler for FlutterAppDelegate.
By default, taps are passed to the key window's rootViewController if
it's a FlutterViewController. Apps with custom app delegates can
customize this behaviour to send the tap to the FlutterViewController(s)
they prefer by overriding touchesBegan:withEvent.

* [iOS] Ensure FlutterMain is called before interacting with the engine in any way.

* Licenses

The PlatformView superclass constructor was posting a task to the UI thread
that adds the view to the shell's global list.  This could result in UI thread
operations seeing PlatformView instances that are not fully constructed and do
not yet have an engine.

This was happening in https://github.com/flutter/flutter/issues/7735

On iOS, when a tap is detected in the status bar, provide a means to
pass that touch event through to one or more FlutterViewControllers to
trigger a scroll to top. In iOS apps, scroll to top should occur under
the following conditions:

1. There is one and only one UIScrollView visible with
   scrollsToTop == YES.
2. The status-bar is in standard height mode, not in double-height mode.
   In double-height mode, the expected behaviour is to trigger a switch
   to the application associated with the double-height status bar.
3. A tap or a drag gesture occurs that is entirely constrained to the
   status bar frame. (We currently only handle the tap scenario).

Unfortunately, AppDelegates only get touchesBegan events for status bar
taps, though get get touchesBegan and touchesEnded events for drags
within the status bar frame. As such, we currently synthesise the
touchesEnded event for taps.

All samples and the 'flutter create' command now use a project-specifc
app delegate.

Any projects that still rely on FlutterAppDelegate should create an
empty delegate that subclasses UIResponder<UIApplicationDelegate> and
defines:
  @property (strong, nonatomic) UIWindow *window;

* Apply iOS status bar padding only to fullscreen views

Previously padding was applied to account for the status bar, whether in
standard or expanded 'in-call' geometry only if the current view was not
a subview of a containing view. This didn't cover the case of root views
embedded in other windows (e.g. dialogs).

We also ensure that the window is fullscreen to account for cases like
small dialogs centred on the screen.

* Do not apply padding if we're a subview in a containing view

* Handle double-height status bar on iOS

In certain cases, iOS displays a double-height status bar (e.g., when an
application is using device location or while in a call). In such cases,
iOS offsets the app view origin by 20px, reduces view height by 20px,
then overlays a 40px opaque status bar: 20px covering the newly opened
20px gap at the top of the screen, 20px covering the top 20px of the
view, which had previously been under the standard-sized status bar.

Flutter previously set top padding to the height of the status bar,
which resulted in 40px padding with a double-sized status bar. However,
the padding should match the portion of the status bar overlapping the
view, which is 20px.

Note that the final case is the one in which no status bar is shown and
padding should be zero.

* Only apply status bar padding on root views

These are implemented on macOS and Fuchsia.

After this patch, we no longer link with Mojo. We still use some gn definitions
from //mojo to create Dart packages.

We no longer use mojom to transport messages. We still use the Mojo EDK
to spin the event loop, however.

Now we just pass the data directly.