AccessibilityBridge support for edge triggered semantics (iOS + Android) 

Use weak pointers to the accesibility bridge from objects vended to the UIKit accessibility framework. (#4761)

* fix iOS crash in a11y mode when used too fast

* clang-format

* implement iOS text field editing in a11y mode

* address Chinmay's comments

* replace node in child list when changing type

This reverts commit a48b5f6d.

Refactoring related to use of the embedder APIs on macOS
host builds.

* Three finger a11y scrolling for iOS

* review comments

* review feedback

* format

* license script adaptions

* updated licenses

* review comments

* Retain the parent of a SemanticsObject

This fixes the last known crash in iOS accessibility.

* rework memory management

* review comments

* Fix crash in iOS accessbility

Previously, whenever an action would remove the focused element from the screen the app would crash. This change tells iOS to focus the first on-screen element after every change to the semantics tree. This avoids the crash.

In a future iteration, we should tell iOS which element it needs to focus by looking at the sematnics tree (instead of leaving it up to the iOS).

* Better handle the case were the focused element stays on the screen

* review comments

* 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.

This patch hides the base::SingleThreadedTaskRunner, which pulls a big
switchover to ftl::WeakPtr and std::function.

Previously we enabled accessibility unconditionally on iOS, which is more
expensive than necessary. We still enable it unconditionally on the simulator
because there's no API for determining whether accessibility is needed on the
simulator.

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.

* Wire up accessibility scrolling

flutter/flutter#1664

* Move node implementation to Objective C

This changes AccessibilityBridge::Node (C++) to be AccessibilityNode (objc)
in order to have the nodes returned directly as ui accessibility elements.
This buys us a few things:

1) We can now return an element tree instead of a flat list, thus negating
   the need to maintain a global rect and global translation
2) We use less memory since we don't need to instantiate both nodes and
   their ensuing accessibility elements
3) accessibility elements can now implement the methods necessary to respond
   to events like user scrolling

https://github.com/flutter/flutter/issues/1664

All the headers for Flutter.framework are now in framework/Headers and all the
source files for Flutter.framework are now in framework/Source. Previously it
was unclear that FlutterAppDelegate and main_ios.mm weren't part of
Flutter.framework. (They still build as part of the framework, but that's
something I'll fix in a later patch.)

Report Flutter widgets to iOS accessibility API