Zen: Ringer mode integration fixes.

 - Properly mute media + alarm streams in total silence.
 - Allow system components to update the "ringer mode affected"
   streams dynamically.
 - Improve dumpsys output for ringer mode streams (include names).
 - Remember last ringer mode properly when entering dnd via a
   volume down (so we can restore it properly when exiting).

Bug: 20854925
Bug: 20950463
Bug: 20756527
Change-Id: I80afc1f639ff802c541020e25ade1b23e9997ef9