[Linux] Reset keyboard states on engine restart

[dkwingsmt]

With this PR, the state of the keyboard system will be reset when the engine is reset (typically during a hot restart.)

Fixes flutter/flutter#89729 and the linux part of flutter/flutter#91064.

Exemption from testing

I don't think we can test FlView within reasonable effort.

• 80% of FlView's code is related to GtkWidget, which can't be run headlessly without bootstrapping the entire GTK environment (which might still be possible, but honestly too much to research given my current task queue.)
• I tried to isolate the rest of FlView's code into a headless class, but it turns out to be huge amount of boilerplate with like 10 lines of effective code (creation, connection, restarting.)

Request to test has been filed at flutter/flutter#91345.

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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[Hixie]

Do we have any end-to-end tests for GTK desktop builds that we could sneak some keyboard testing into?

cc @cbracken @robert-ancell @stuartmorgan

In the absence of any reasonable way to test this:
test-exempt: untestable

Please file a bug noting describing the scenarios we should test, though, if you don't check in tests here.

[cbracken]

Most of the text editing testing we have is either unit tests for text editing or keyboard input channel or written against text editing APIs and implemented with a dart test harness.

We don't currently have test harness abstractions for integration tests that would allow an engine restart and a way to send key events, but I can imagine writing a new class of devicelab test harness that would support such a thing.

I'll file a bug.

[gspencergoog]

[dkwingsmt]

Do we have any end-to-end tests for GTK desktop builds that we could sneak some keyboard testing into?

cc @cbracken @robert-ancell @stuartmorgan

In the absence of any reasonable way to test this: test-exempt: untestable

Please file a bug noting describing the scenarios we should test, though, if you don't check in tests here.

Filed at flutter/flutter#91345