Bug 5567648 disassociate audio mode and audio focus

Don't automatically change the audio focus when
 the audio mode changes. This is best handled by the
 applications that change the audio mode so they
 can address their usecases as they please (for
 instance to define the behavior when switching calls).
Replaced the implicit "mode to focus" behavior with
 two methods to request and abandon audio focus. These
 methods are only to be used by the framework, and maintain
 the logic in AudioService to prevent other apps to request
 audio focus during a call.
A susequent change will update com.android.internal.telephony.CallManager
 to take advantage of these two methods.

Change-Id: If84ebd508e985083e8cac82ece44940c72b5c669