[audio] Add playback hardening for MMAP

Silence tracks who don't have OP_CONTROL_AUDIO_PARTIAL, which requires
MODE_FOREGROUND, for MMAP tracks.

Test: Oboe Tester MMAP background playback before and after
Test: Alarm playback after kill
Test: Spotify playback continues
Test: Spotify after kill resumes from UMO, media event
Test: Meet ringtone
Test: WA ringtone
Bug: 376481063
Flag: com.android.media.audio.hardening_impl
Change-Id: I91c8e41fa5a8dd52b5062631ae2047a062945a63