[WIP] Support iOS Premotion refresh rate

[luckysmg]

This PR lets engine can change the screen's refresh rate (only iOS now).The framework will call window.updateFrameRate(frameRate) to let engine to update frame rate.

This solution is functionally feasible, but needs further discussion with the flutter team

The flutter framework PR:flutter/flutter#96710

List which issues are fixed by this PR. You must list at least one issue.
flutter/flutter#90675

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• 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 ///).
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• All existing and new tests are passing.

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[flutter-dashboard]

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