Return of Silent mode (1/2)

- Adds ability to replace "None" with "Silent" in the Volume panel

- Right now, it will behave exactly the same as "None",
except allow alarms to go through

Change-Id: Icee44a7114e9529d139c52e5e37e3ec747cec336
Signed-off-by: Roman Birg <roman@cyngn.com>