This PR implements thread merging for both lightweight multiple engines and standalone multiple engines.

Fixed issues:
flutter/flutter#73620
flutter/flutter#78946

The design of this implementation:

• This change makes sure to be platform independent. Android and iOS share the common logics and the design attempt to ensure that the code in the relevant directory of the different platforms has not been changed.
• Removed the codes of block thread merging logics to let multiple merging allowed (aka removed this change: block thread merging with shared engines #23733)
• Make the member owner_of in TaskQueueEntry from a TaskQueueId to std::set<TaskQueueId> owner_of to record all subsumed of the owner.
• In order to reduce the changes count of existing code, I regard the old raster_thread_merger class as a handler (or a proxy) and introduced a new shared_thread_merger class, use owner_id and subsumed_id as the key of the map, and values are the shared instances of shared_thread_merger class.
• Merge related method calls including MergeWithLease(), UnmergeNow(), DecrementLease() , IsMergedUnsafe() was redirected to the methods insideshared_thread_merger, so it can take the same lease_term_ counter to determine merge state.
• A new method RecordMergeCaller() was added and changed UnMergeNow() to UnMergeNowIfLastOne() to remember all merge callers and in order to "unmerge immediately" only if the merger is the last one, when Rasterizer::Teardown() was called.
• Added some more tests in shell_unittest and fml_unittests, and temporarily uncomment and enablded the fml_unittests in run_tests.py

Merging cases for lightweight engines and standalone engines:

Allowed and disallowed merging operations:

My demos:

1. Lightweight engines (it merges same gpu thread into one platform thread) : Using this sample and add a flutter animation and a webview inside main.dart
   Link: https://github.com/flutter/samples/tree/master/add_to_app/multiple_flutters/multiple_flutters_android

View screen record video below:

I found if it uses FlutterFragment and make the surfaces of multiple flutter instances not rendering properly.
Then the code in FlutterSurfaceView.java will call the setZOrderOnTop()

  private void init() {
    // If transparency is desired then we'll enable a transparent pixel format and place
    // our Window above everything else to get transparent background rendering.
    if (renderTransparently) {
      getHolder().setFormat(PixelFormat.TRANSPARENT);
      setZOrderOnTop(true);
    }

I added .transparencyMode(TransparencyMode.opaque) to make it render again. Firstly I wrote the code of multiple thead merger and found this issue. It has no relationship of
In https://github.com/flutter/samples/blob/master/add_to_app/multiple_flutters/multiple_flutters_android/app/src/main/java/dev/flutter/multipleflutters/DoubleFlutterActivity.kt

            val flutterFragment =
                FlutterFragment.withCachedEngine(i.toString())
                    // Opaque is to avoid platform view rendering problem due to wrong z-order
                    .transparencyMode(TransparencyMode.opaque) // this is needed
                    .build<FlutterFragment>()

2. Standalone engines (it merges multiple gpu threads into one platform thread)
   Link: https://github.com/eggfly/unable_to_merge_raster_demo
   This code is from the issue: Two webviews in separate flutter engines and show in one page will cause native crash. flutter#78946
   

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

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