Run audio at priorities 2 and 3 above kernel 1

Also run the watchdog at same priority as the fast mixer.
requestPriority() originally used only the caller's pid to decide which
cgroup to assign, but in the future it might look at the priority also.
So it's safer to use same priority as the fast mixer to be sure we
run in the same cgroup.

Bug: 6461925
Change-Id: Ia59c93e4b22dacbb6746bfa6ad491be7b72f2b8d