Update AudioService to reflect phone state changes with AudioFocus.

Phone calls and rings are treated as a new client, with a dedicated
client ID that identifies it in the audio focus stack. When it is
present in the stack, others clients cannot request audio focus
(request is denied).
Because the phone state is now mostly handled like a new client,
there is no need monitor mode changes and to keep track of
undispatched focus changes, as those will delayed focus gains
will now automatically happen at the end of the call when the
phone client is removed from the focus stack.

Change-Id: I11ff73b015ab93f07040755fd8ee75c8d675e025