Have AudioService listen to DISALLOW_UNMUTE_MICROPHONE and

... DISALLOW_ADJUST_VOLUME, instead of UserManager pushing
new settings to AudioService.

Also:
- Allow PO to set these two restrictions.

- Now AS.setMasterMuteInternal() respects mUseFixedVolume to make
it consistent with readPersistedSettings().

- When a user switches and restores the mute state in
AS.readPersistedSettings(), also check the current user restrictions
in addition to system settings. Because of the delay in AudioService
before persisting the mute settings in setMasterMuteInternal() and
setMicrophoneMute(), there's was an edge case
DISALLOW_UNMUTE_MICROPHONE and DISALLOW_ADJUST_VOLUME would be ignored
when the user switches right after they are set.

Bug 24981972

Change-Id: I4d9b709a0a0e6812319204568c6e44d6664bdeb4