Making mpv the Android default only moved installs that had never opened
the Player Backend setting. Anyone who had ever picked ExoPlayer kept it,
including the low-end TVs and Hi10P devices the new backend was built for.
The backend choice now lives under a new `android_use_exoplayer` key and
the old `use_exoplayer` one is dropped at startup, so every install starts
on mpv. ExoPlayer stays selectable in Settings > Playback > Player Backend,
and a pick made there writes the new key and sticks.
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