Allow Bluetooth sys_nice and system_server setsched for Bluetooth HAL

Bluetooth needs the capability to set audio-related threads to be RT
scheduled.  Grant it sys_nice.

system_server needs to set priority for the Bluetooth HAL.  Allow it.

Bug 37518404
Test:  Play Bluetooth audio, confirm RT scheduling with systrace
Change-Id: Iaf7b85a11a51883744d72a50addfd320b6fbbc2f