Add @RequiresPermission annotation for Call#enterBackgroundAudioProcessing()

Permissions needed are CAPTURE_AUDIO_OUTPUT and MODIFY_AUDIO_ROUTING. Intdef annotations for @AudioProcessingUseCase are also being add to Call#enterBackgroundAudioProcessing() and
Connection#getAudioProcessingUseCase() parameters and return values.

Flag: com.android.server.telecom.flags.enable_audio_processing_use_case
Test: cts tests
Bug: 425684919
Change-Id: I546f640c3104a95c20164bf35c3e9162aa357ebe