Add shell command to toggle DND

adb shell cmd notification set_dnd [type]

Where type is one of:
none (total silence mode)
on (total silence mode)
priority (priority only mode)
alarms (alarms only mode)
all (turn dnd off)
off (turn dnd off)

Test: manual
Fixes: 132901740
Change-Id: I9b634570a0bd6dd0a3277268e39a8ca67e177e89