Alarm: Show a warning toast when alarm volume is set to silent

Change-Id: I97edae55351101046def5058a2459ab88edf2d0d
DeskClock: Request user to confirm power-on device on power-off alarm event

Show a dialog to request user confirmation to power on the
phone when power-off alarm pops up and user dismisses it.
On user confirmation power-on the phone.Otherwise,keep the
phone switched off.

CRs-fixed: 637275

Change-Id: I20250fa3a86bc51cfaa3bd8b26906755f2726556

(cherry picked from commit ec6a9fd46dccfabc2d0fde71a88b7535d0edfc80)