[video_player] Update Android to exoplayer 1.9.1

[stuartmorgan-g]

Updates from 1.8.0 to the just-released 1.9.1.

ExoPlayer 1.9 is no longer trivially mockable, so I followed the team's general guidance to use a real ExoPlayer in a Robolectric test instead of mocking it.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
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• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
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• I updated/added any relevant documentation (doc comments with ///).
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• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩↩²↩³

[stuartmorgan-g]

Oddly we don't seem to have gotten a dependabot PR for the 1.8.0->1.9.0 update.

[gemini-code-assist]

Code Review

This pull request updates the androidx.media3:media3-exoplayer dependency to version 1.9.1. As part of this update, the unit tests have been refactored to use Robolectric with real ExoPlayer instances, since mocking is no longer straightforward with the new version. The changes are consistent across the build files, changelog, and tests. My feedback focuses on ensuring proper resource cleanup in the updated tests.