Improves the changes made in 57687779 by
using AudioManager.ACTION_AUDIO_BECOMING_NOISY rather than listening for
wired headset unplug events or Bluetooth headset disconnect events. This
method is more flexible, allowing the feature to work on other types of
audio output device, as well as Bluetooth devices that don't set their
device class correctly. This change also has the benefit of being more
responsive, pausing the audio before it is rerouted to the built-in
speaker.

https://developer.android.com/guide/topics/media-apps/volume-and-earphones

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Fixes assertion failure when the submit queue is empty.

Fixes MixRamp failures when a MP3 file has two ID3 tags, one of them
without MixRamp.

Fixes regression by commit 8e0d8109 which is 2 years old, and nobody
noticed.  D'oh, how embarassing!