Fixes flutter/flutter#88915

Overriding the NSView method acceptsFirstMouse will allow the initial mouse-down event when the window is not focused to pass to the view.

Since not all developers may want this, this could ideally be a settable control, but this behavior at least matches Windows. It would also be possible for a developer to simulate this behavior in their application.

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.

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