Bluetooth: Boost timer callback thread priority to high

- Boost timer callback thread priority to high for
  getting alarm callback with priority during a2dp
  music playback.

- Media task/btu/hci/event threads are all high priority
  but actual callback thread is remaining normal which
  mean it's possible not to schedule alarm callback
  in-time if system is busy.

CRs-Fixed: 935097
Change-Id: I10dff46af800d55f0e816353a0f39cfe3150d22f
3 files changed