[Media ML] Replace AudioSystem.isActiveStream

AudioSystem.isActiveStream is a hidden API. Replace it with
getting all audio playback configurations using
AudioManager.getActivePlaybackConfigurations and checking if the
given stream exists among them.

Bug: 169386435
Test: Manually checked that volume key works properly while
alarm/music audio is active

Change-Id: I096b28a741353b7a3a93f7e6a73177d29b6d2748