vibrator: Add a way to run the vibrator with reduced priority

 * The current vibrator implementation runs with URGENT_DISPLAY
   priority. This is normally what you want- keyboard and input
   haptics should be fast. In the case of an incoming call ringtone
   though, it's possible for the high priority thread to cause
   starvation if the device is under heavy load. This can result
   in a missed call.
 * Add a hidden API which lets us call the vibrator at
   FOREGROUND-1 priority which will be used from Telecomm for
   this use case.

CYNGNOS-969

Change-Id: Id0770218e1f39cffaeaf170531ae8f3fc3884eee