[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

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• I read and followed the relevant style guides and ran the auto-formatter.
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• I linked to at least one issue that this PR fixes in the description above.
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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.