Upload screenshot snapshots to Sentry (#5378)

* feat(android-core): Upload screenshot snapshots to Sentry

Replace local golden-image comparison in ScreenshotEventProcessorTest
with Sentry Snapshots for visual diffing. Screenshots are now generated
to build/test-snapshots/ and uploaded via sentry-cli in CI.

- Remove local snapshot comparison logic and dropbox-differ dependency
- Delete golden images from version control
- Add sentry-cli install and upload steps to build.yml
- Use PR head SHA checkout for correct base-vs-head diffing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(ci): Use correct Sentry org and project for snapshot upload

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>