Audio hardening: audio focus requires control capability

Add audio focus request to AudioService's HardeningEnforcer
filtering.
Requesting audio focus requires being performed from a
foreground state or service, and is mapped to a new
capability, FOREGROUND_AUDIO_CONTROL.
Logging for focus hardening goes to system log, add
methods in EventLogger for Slog output.

Bug: 296232417
Test: adb shell device_config put media_audio android.media.audio.foreground_audio_control true ;\
      atest CtsAppTestCases:android.app.cts.ActivityManagerFgsBgStartTest ;\
      atest AudioFocusTest

Change-Id: I2abc27c2439af77690685f779998c0fdfa1f77f0