Only schedule mouse tracking callback when there is a mouse

[dkwingsmt]

Description

PR #44631 introduces a post-frame check that is scheduled after every frame, which affected some benchmarks (namely stock_layout_iteration).

After this PR, the post-frame check is only scheduled when there is a mouse connected, which hopefully improves the performance. (Through my local testing there is ~1% improvement)

Related Issues

• Improve MouseTracker lifecycle: Move checks to post-frame #44631 (comment)

Tests

I added the following tests:

• should not schedule postframe callbacks when no mouse is connected

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.

[gspencergoog]