Flag for FGS audio control

Flag controls ability for apps to request
audio focus gain, requires app to be
foreground or FGS or delegate for mic / call /
audio playback / camera service type.

Flag: android.media.audio.foreground_audio_control
Bug: 296232417
Test: adb shell device_config put media_audio android.media.audio.foreground_audio_control true ;\
      atest CtsAppTestCases:android.app.cts.ActivityManagerFgsBgStartTest
Change-Id: Ic6c240e2ba8156220f7af7cf8c6a405384beca64