Reverts the revert in #4448 with fixes to pass on the bot.

This change will require framework changes in flutter/flutter#13490.

This reverts commit 59c3a37e.

* a11y cursor movement by character

* focus fixes

* doc fixes

* Identify text fields as such to a11y on Android

* Add focus flag

* Android SeekBars announce their value

* style

* Support for accessibility label and hint

* review comments

https://github.com/flutter/flutter/issues/11992

This ensures that accessibility hints are played properly.

* Various iOS a11y fixes

* undo

* review comments

* review comments

A node is considered focusable if it contains information that is interesing to the user. A node that doesn't add any semantic information of its own should not be focusable. It's expected that such a node has children, who have smeantics information and are therefore focusable.

Fixes https://github.com/flutter/flutter/issues/11179

* order

* comment fix

* review comments

* Only expose available scroll actions

* formatting

* more formatting

This action is triggered when the user swipes (in accessibility mode) to the last visible item of a scrollable list to bring that item fully on screen.

iOS implementation to follow.

See also https://github.com/flutter/engine/pull/3701

Fixes https://github.com/flutter/flutter/issues/10855

* Adding SemanticsFlag.isSelected

* typo

The core problem was multiplying matrices the wrong way around.

While I was there I did some minor cleanup. I think that may have
fixed another bug relating to A11Y turning on or off improperly, but
since I never figured out the steps to reproduce, it's hard to tell.

Fixes https://github.com/flutter/flutter/issues/7205
Fixes https://github.com/flutter/flutter/issues/10059 (maybe)

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

TalkBack still isn't fully working, but this patch gets us much closer.

* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.

These classes now use the same terminology and work in the same way.
Also, change semantics.mojom to use an enumeration of actions instead of
having a separate method per action. This will hopefully scale better.

Fix crashes with LastPass, Now-on-Tap, DashLane

This patch cleans up the public Java interface to the Flutter engine. The code
that we intend to be re-usable is now in the io.flutter.view package. The two
public classes are:

 * FlutterMain, which controls initialization of the system, and
 * FlutterView, which is an Android view that hosts a Flutter app.

We'll need to interate on these interface over time, but at least these names
are more reasonable.

- Correctly enable accessibility on startup if it's already enabled.
- Avoid using deprecated APIs.
- Map FORWARD/BACKWARD scrolling actions to up/down even when left/right
  is also possible.
- Clear the accessibility focus when removing a focused semantics node.
- Unregister listeners when view goes away.
- Call setWillNotDraw(false) if accessibility is enabled.
- Clean the handling of hover events.
- Work around an Android bug where the system focus rect isn't drawn
  the first time.

If we send accessibility events when accessibility is disabled, we crash.