Fix problems in tablet silent mode.

Do not enter silent mode when ALARM stream volume is changed
to 0 by volume down key: Only RING, NOTIFICATION and MUSIC
streams control silent mode.

Report correct volume (0) for NOTIFICATION stream when silent mode
is entered by changing NOTIFICATION stream volume to 0 with
volume down key.

Change-Id: I3e0816dfae40bc127cc30cca02cdca6ec19e30a4