Add new audio mode for audio communications other than telelphony.

The audio mode MODE_IN_CALL signals the system the device a phone
 call is currently underway. There was no way for audio video
 chat or VoIP applications to signal a call is underway, but not
 using the telephony resources. This change introduces a new mode
 to address this. Changes in other parts of the system (java
 and native) are required to take this new mode into account.
The generic AudioPolicyManager is updated to not use its phone
 state variable directly, but to use two new convenience methods,
 isInCall() and isStateInCall(int) instead.

Change-Id: Id744cd26520ea1d1a4795eabe6a1f0c58789af76