[fuchsia] Implement KeyboardInputListener::ReportReady

[filmil]

The implementation of a FIDL server side requires implementing all methods.

Issue: https://github.com/flutter/flutter/issues/112991

This will need to wait until the new Fuchsia SDK rolls into Flutter.

Pre-launch Checklist

• [X] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [X] I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• [X] I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• [X] I listed at least one issue that this PR fixes in the description above.
• [X] 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 ///).
• [X] I signed the [CLA].
• [X] All existing and new tests are passing.