Remove UserManager.setSystemControlledUserRestriction()

Now that we don't have UM.setUserRestriction*s*() that could remove
all existing restrictions, there's almost no point handling
DISALLOW_RECORD_AUDIO differently.

Now DISALLOW_RECORD_AUDIO is handled just like other restrictions,
except we don't persist it.

Bug 24954662

Change-Id: I27875b4a74dd95a3ce6bb774081eeaf718eaec15