Fix memory leak and bug in the RunsOnCreationTaskRunner check

[blasten]

I encountered these issues while debugging customer:money app.

1. Memory leak:

┬───
│ GC Root: Global variable in native code
│
├─ android.database.ContentObserver$Transport instance
│    Leaking: UNKNOWN
│    ↓ ContentObserver$Transport.mContentObserver
│                                ~~~~~~~~~~~~~~~~
├─ io.flutter.view.AccessibilityBridge$3 instance
│    Leaking: UNKNOWN
│    Anonymous subclass of android.database.ContentObserver
│    ↓ AccessibilityBridge$3.this$0
│                            ~~~~~~
├─ io.flutter.view.AccessibilityBridge instance
│    Leaking: UNKNOWN
│    ↓ AccessibilityBridge.platformViewsAccessibilityDelegate
│                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
├─ io.flutter.plugin.platform.PlatformViewsController instance
│    Leaking: UNKNOWN
│    ↓ PlatformViewsController.overlayLayerViews
│                              ~~~~~~~~~~~~~~~~~
├─ android.util.SparseArray instance
│    Leaking: UNKNOWN
│    ↓ SparseArray.mValues
│                  ~~~~~~~
├─ java.lang.Object[] array
│    Leaking: UNKNOWN
│    ↓ Object[].[0]
│               ~~~
├─ io.flutter.embedding.android.FlutterImageView instance
│    Leaking: YES (View.mContext references a destroyed activity)
│    mContext instance of *****.app.MainActivity with mDestroyed = true
│    View#mParent is set
│    View#mAttachInfo is null (view detached)
│    View.mWindowAttachCount = 1
│    ↓ FlutterImageView.mContext
╰→ *****.app.MainActivity instance

2. RunsOnCreationTaskRunner/FML_DCHECK_TASK_RUNNER_IS_CURRENT check had a bug if thread merging is enabled. The unit test shows the bug.

3. Added leakcanary to start performing memory leak tests in the scenario app. (Will follow up with more instructions on how to use it as I learn more).

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.
• The reviewer has submitted any presubmit flakes in this PR using the engine presubmit flakes form before re-triggering the failure.

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