Enhance MediaRepository with reactive flow and update ImageWell animation

[temcguir]

This PR refactors the media data layer to support a reactive unidirectional data flow, enhances the capture UI with improved animations, and addresses b/522926012 by supporting dynamic file prefixes.

Core Changes:

• Reactive MediaRepository: Updated LocalMediaRepository to expose a lastCapturedMedia StateFlow. It now automatically initializes with the latest app-captured media and reacts to MediaStore changes.
• Thread Safety: Added a Mutex to LocalMediaRepository to synchronize access to the media cache and prevent race conditions during concurrent MediaStore updates.
• Dynamic File Prefix (Fixes b/522926012): Exposed a prefix property in FilePathGenerator. LocalMediaRepository now uses this prefix dynamically when querying the MediaStore, allowing it to correctly identify app-captured media even when a custom generator (e.g., for Google Photos) is used.
• Refined Ownership Check: Updated the repository to strictly use OWNER_PACKAGE_NAME on Android Q+ for identifying app-specific media, falling back to the filename prefix only on older versions.
• Enhanced ImageWell UI: Replaced the static thumbnail display with an AnimatedContent transition that performs a "vertical push" animation whenever the last captured media updates.
• Simplified Testing: Refactored FakeMediaRepository to use a direct StateFlow, avoiding unmanaged coroutine scopes and potential leaks in tests.

Verified:

• Built :data:media and :ui:components:capture successfully.
• Formatting verified with spotlessApply.

[gemini-code-assist]

Code Review

This pull request refactors the media repository to expose the last captured media as a continuous StateFlow (lastCapturedMedia) instead of a suspend function, and updates the ImageWell UI component to observe this flow with smoother transition animations. It also enhances the testing infrastructure, including FakeContentProvider and LocalMediaRepositoryTest. The review feedback identifies a potential race condition in LocalMediaRepository due to unsynchronized cache access, suggests a more robust package ownership check on Android Q+, and recommends simplifying the flow implementation in FakeMediaRepository to prevent coroutine leaks.