Refactor FrameTimingsObserver for multi-activity support

[rubennorte]

Summary:
Changelog: [internal]

Previously, FrameTimingsObserver required a Window at construction time and could not track frames across activity changes. In multi-activity Android apps, navigating between activities would cause frame timing to stop being recorded from the new activity's window.

This refactors FrameTimingsObserver to use a push-based setter pattern where ReactHostImpl calls setCurrentWindow() whenever the current activity changes (in onHostResume and moveToHostDestroy). The observer re-registers its frame metrics listener on the new window automatically.

Key changes:

• Remove window from constructor, add setCurrentWindow() method
• Add isStarted flag to track observer state for proper listener registration on window changes
• Mark currentWindow as Volatile for thread safety (UI thread writes, background coroutine reads)
• Extract registerFrameMetricsListener() / unregisterFrameMetricsListener() helpers to reduce duplication

Differential Revision: D94358266

[meta-codesync]

@rubennorte has exported this pull request. If you are a Meta employee, you can view the originating Diff in D94358266.

[react-native-bot]

This pull request was successfully merged by @rubennorte in 8230f3b

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