Play a short/silent WAV file to ensure we get routed media key events

Bug: b/112431289
Test: Pair BT phone and play music. Use adb shell input keyevent
to send all of 126/play, 127/pause, 86/stop, 88/prev, 87/next,
164/mute, 25/volume-down, 24/volume-up. Stream and functions behave as
expected. Additionally, check that dumpsys media_session shows bluetooth
as both the MediaButtonSession AND a valid audio playback source.

Change-Id: Ic9d096f4b372f3fd45448d01d953e8d38e7b2278
Merged-In: Ic9d096f4b372f3fd45448d01d953e8d38e7b2278