Mute correct user from device policy manager.

Add per user versions of mute methods so
device policy manager can mute the correct
user.
Just persist change if the calling user
isn't the current user.
Treat calls to audio manager coming from uid
1000 as if they were coming from current user
rather than user 0 so that the correct user's
user restriction is checked.

Bug: 21782066
Bug: 21778905
Change-Id: I51469b741096d8a2ffdc520eaf5b3fd754f2c819