Unhide android.media.AudioManager.MODE_IN_COMMUNICATION

Unhide new constant for an audio mode for a system state when
 the device has a VoIP (as opposed to telephony) call established.
Update the setMode and getMode javadoc to link to the mode constants
 that can be set or retrieved.

Change-Id: I90c4417679f215eb66e5305086fd077beddaa59c