Mark the Flutter Views as focusable by setting a tabindex value.

[tugorez]

Mark the Flutter View as focusable by setting a tabindex value.

• When a given flutter view is focused its tabindex will be -1
• When a given flutter view is not focused its tabindex will be 0
• When semantics are enabled no tabindex will be set.

Relevant Issues are:

• Design doc: https://flutter.dev/go/focus-management
• Focus in web multiview: [web:multi-view] Make keyboard and focus multi-view aware flutter#137443
• Platform dispatcher changes: Add the focus state related methods to the platform dispatcher #49841

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 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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