This PR makes KeyboardManager an all-in-one component that encapsulates the entire keyboard system, and reworks the testing framework to support such changes.

All of KeyboardManager's external dependencies are now extracted as KeyboardManager.ViewDelegate, which is implemented by FlutterView. The keyboard responders are now created and managed by the manager, instead of the view. In this way, the view is no longer aware of any implementation details, but only what the manager needs from outside. Moreover, what's needed for KeyboardManager by InputConnectionAdaptor has also been extracted as a delegate.

Moreover, the testing framework has been reworked. The new framework mocks and compares the end result of the manager's effects, i.e. the engine and the messenger. It also adopts the latest "class-based" design.

As a non-trivial change, the channel message no longer provides vendorId and productId. Mocking them is a little harder under the new test framework, and they're likely not used much, and are expected to be removed in KeyEvent anyway.

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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