* Convert semantics_node from SkMatrix44 to SkM44

* "Reland Clang 11, roll buildroot to 1bc40a5f916adb93d87e2a2230a0ec43db876c1c, take 4 (#17467)" (#17482)"

This reverts commit 8166a91b.

* format code

* Roll buildroot to 3a27de1c5bb0f50be7d3efe3e00de4f6068c9f30

* Guard against orphaned semantic objects trying to reference a dead bridge on iOS.

* Switched back to a function instead of a macro for checking the bridge.

* Fixed some formatting issues.

* Fixed the scroll direction for horizonatal accessibilty scroll events.

* Updated the comment describing the scroll direction mapping.

This reverts commit 974ca210.

Made the engine and the view controllers have BinaryMessengers, not be
BinaryMessengers.  This allows us to break retain cycles and makes the
leaking channels we have not less dire.

Partially fix the issue in flutter/flutter#30804. This only fixes the focus issue for other platform views (Google maps and other custom platform views), but the same issue on WebView is not fixed.

Before the change, the a11y tree with platform view looks like

```
<some parent a11y container>
     <SemanticsObject for platform view>
     <FlutterPlatformViewSemanticsContainer>
          <platform view>
```
After the change, it looks like

```
<some parent a11y container>
     <FlutterPlatformViewSemanticsContainer>
          <SemanticsObject>
          <platform view>
```

This PR also updated the implementation of FlutterPlatformViewSemanticsContainer to use A11yContainer protocol to implement the details on various a11y attribute including the accessibilityFrame and accessibilityScroll.

Some components in the Flutter engine were derived from the forked blink codebase. While the forked components have either been removed or rewritten, the use of the blink namespace has mostly (and inconsistently) remained. This renames the blink namesapce to flutter for consistency. There are no functional changes in this patch.

Follow up the framework change in flutter/flutter#29304.
Inject the accessibility element tree in the semantic node if the node is for platform views.

flutter/flutter#29302

* fix memory leak in a11y bridge

Fixes https://github.com/flutter/flutter/issues/22206

* Create FlutterEngine to manage a Shell to support maintaining execution state across instances of FlutterViewControllers
* Refactor PlatformViewIOS to support adding or removing a FlutterViewController dynamically
* Refactor FlutterDartHeadlessCodeRunner to implement FlutterEngine
* Refactor FlutterViewController to accept a FlutterEngine at initialization or to create one for backwards compatibility; any Shell related responsibilities are now passed through to the Engine instance
* Remove FlutterNavigationController (unused)
* Update all public Objective C doc comments to be consistent and formatable
* Add public documentation to all public headers

* Enforce clang-format on all files in commit

This re-enforces clang-format across all files changed in the commit.

In c10c4173, we enabled checking only for the lines changed in the diff
in order to reduce the change of merge conflicts with the shell refactor
landed in 58e84c8b.

* Reformat sources to match latest clang-format

As part of re-enabling clang-format across the codebase, reformat all
code to match the latest toolchain.

Not all platforms are expected to handle all SemanticsEvents. Therefore, it is ok to just drop unsupported events on the floor.

* split `children` into two ordered lists: traversal and hit test

* address comments

* reduce node object byte size

* link to DebugSemanticsDumpOrder

AccessibilityBridge support for edge triggered semantics (iOS + Android) 

* Re-land "Support multiple shells in a single process. (#4932)"

This reverts commit 723c7d01.

This reverts commit a3327bff.

This reverts commit 9199b40f.

This reverts commit 6baff4c8.