Relands #33113, which was reverted due to possible internal breakages.

Changes based on the original PR: (See latest commits)

• Fix lint errors
• Additional documentation.

This PR implements the embedder keyboard responder of the Android embedding, the backend of HardwareKeyboard on Android. This is the last platform to support it.

The behaviors are tested on a ChromeOS over ARC++. There are some weird behaviors, mostly for metaState, which I'm not sure is the fault of ARC++ or Android. I don't have an Android device with keyboard to test for now. Therefore the synchronization logic is implemented in a loose way.

There are a few known issues:

• I haven't had chance to test the character combination yet (due to the next item). I just used the same mechanism of the channel embedder.
• [Android] After focusing on a text field, no printable key events can be received ever flutter#104031

The framework PR flutter/flutter#104032 generates the necessary codes for this PR.

Related Issues

• Umbrella: ☂️ Device-specific keyboard events should happen in the embedder flutter#44918

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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