Fix issue 4673378: switching from VoIP to GSM call

The problem is that any app can change the audio mode and override
a mode previously set by another app. If two apps (gTalk and Phone) compete
for audio mode ownership, there is no mechanism to maintain coherency
in the actual audio mode selected.

Added a mechanism in AudioService to manage an audio mode request stack.
Any app requesting a mode different from NORMAL enters at the top of the stack
and the requested mode is applied. When an app sets mode back to NORMAL, it exits
the stack and the new mode corresponding to the request at the top of the
stack (if any) is applied.

Change-Id: I68d1755d0922f680df4a19bfc5ab924f5a5d8ccd