Set FrameExtractor to use foreground processing ExoPlayer priority

Currently, `FrameExtractor` spins up its internal `ExoPlayer` using the
default `C.PRIORITY_PLAYBACK` task priority. If the application is
concurrently playing a video, this default priority can cause the system's
resource manager to mistakenly reclaim the active playback codec instead of
the FrameExtractor codec when hardware resources run out.

This CL configures the internal player's `ExoPlayer.Builder` with
`C.PRIORITY_PROCESSING_FOREGROUND` to ensure that the system's resource
manager correctly prioritizes active playback and reclaims the frame
extraction codec first during resource cleanup.

PiperOrigin-RevId: 940461688