Show snackbar when alarm stream volume is muted

Bug: 23940484

- each time the DeskClock activity is brought forward the snackbar
will be displayed if the alarm stream volume is muted
- if the DeskClock activity is already in the foreground and the
alarm stream is muted/unmuted, the snackbar is displayed/hidden
- the snackbar is displayed everywhere; it is not tab-specific
- the snackbar is not displayed after configuration changes
(e.g. rotation)

Change-Id: Ib833be06390d692bb91475fab05f107df8f92f8f