In previous change we use setForceUse(FORCE_MEDIA, XXX) method
in order to set specific audio output type, such as LINE, HDMI_ARC,
and SPDIF. But it turns out that it conflicts with bluetooth
a2dp which uses the same setForceUse(FORCE_MEDIA, yyy).
This change is based on several conditions.
1. When other non-speaker devices are on, prevent system audio
from turing on.
2. In order to keep track of other devices' connectivity and to
turn off system audio if other device like bluetooth or
headphone preempts current output, register OnAudioPortChangeListner
to audio manager.
3. All possible system audio outputs can be merged with other
outputs without priority.
Change-Id: Id4e47d99db64b9f77a17c2c28c47787ab8980bf7
6f34f5ab8a