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.

This lets you focus and unfocus a node.

Still doesn't seem to be enough to actually have the node exposed in
TalkBack. We probably need hover for that (that's next).

(Also fixes a leak for when a subtree has been disconnected; previously
we were only forgetting the top node of the subtree, not the
descendants. Thanks @krisgiesing for catching that.)

Hard to say if this actually works. We'll find out once we've filled
this out enough that you can walk the tree in Android's Talkback UI.

This exposes the actions on AccessibilityNodeInfo nodes.
It also tries to make the code that handles reloads more correct.

This is another step towards enabling accessibility for Flutter on
Android. It exposes the semantics tree to Android's accessibility API
when accessibility is enabled.

It does not yet:
 - allow one to actually interact with the application via the
   accessibility API
 - expose the accessibility tree to touch exploration
 - implement the accessibility focus API

However, you can see the tree if you run uiautomatorviewer. It is there,
and it matches the UI. At least in Stocks. I didn't test anything else.