Add multi user volume management

Notification and Alarm volumes are per user: they are saved and restored
when the foreground user changes.
Media volume is global: it is still saved and restored per user for
implentation reasons but is copied from one user to the next to ensure
media playback volume continuity when switching users.

Ringer mode (silent, vibrate...) is now a global setting.

Bug 7128886.

Change-Id: I9f4f5a0a3985552bca61c2cc3bbe5a144db755a6