Raise alarm callback thread priority

Since alarms [/callbacks] are used in the critical audio path, raising
the thread priority is required to ensure timely encoding/transmission
of audio packets and avoiding thread pre-emption.

This patch indiscriminatly raises the (single) callback thread priority,
affecting all alarm callbacks. In the future, thread priotiry should be
adjusted based on the alarms pending in the queue.

Bug: 24570959
Change-Id: I64928fd7325f1d6f76815a185dbccf1e01ae30d2