[Android] Fix --merged-platform-ui-thread flag

[camsim99]

Specify that the --merged-platform-ui-thread flag can take an argument in the embedding so that it can be set from the manifest (setting via the command line words with or without this PR).

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Code Review

This pull request updates the --merged-platform-ui-thread flag definition in FlutterEngineFlags.java to require an explicit value (appending =), and updates the Javadoc to document that it accepts enabled or mergeAfterLaunch. It also updates existing tests and adds new test cases in FlutterLoaderTest.java to verify the correct parsing and passing of this flag with its values from manifest metadata in both debug and release modes. There are no review comments, and I have no feedback to provide.