AudioService: add test APIs for audio focus and ducking

Note: all new test methods end with "ForTest" so there is
 no ambiguity about the purpose of those methods.

Add test methods to protect focus-based tests from external
 audio focus uses (e.g. NotificationManager playing a
 notification on the DUT during a test).

Add test methods to read ducking-related timing values, and
 the list of ducked UIDs.
Add more debug messages (disabled by default).

Bug: 286495078
Test: atest CtsMediaAudioTestCases:android.media.audio.cts.AudioFocusTest#testDuckedUidsAfterMediaSpeech
Test: atest CtsMediaAudioTestCases:android.media.audio.cts.AudioFocusTest#testDuckedUidsAfterMediaMusic
Change-Id: Ie2fc243eecdc21f3b0718f6392e7345d3fb0ae07