Raise BTU and HCI thread priorities

The audio sub-system and audio related tasks run at elevated thread
priorities in order to ensure timely handling of audio packets. This
patch raises the thread priority of lower layer Bluetooth tasks involved
in delivering audio packets to avoid thread pre-emption and subsequent
audio skipping.

Bug: 24570959
Change-Id: I5e19ee4590207df753f7b8bfc22174959722ec2d