Notification sound-to-vibration conversion toggle

Allows to change the new feature of AOSP 4.2, where all notification are
changed to vibrations on vibrate mode. Instead, allow the old behaviour
to silence sound notifications and to only play vibrations if desired.
Alle credits go to AOKP, I just ported it.

Change-Id: If94a2395f13261eea7b0588f218fc917e8a25a4d