btlinux: Add SYS_NICE to btlinux service

btlinux service is trying to set SCHED_FIFO policy and fails:
android.hardware.bluetooth@1.1-service.btlinux: ThreadRoutine unable to
set SCHED_FIFO for pid 385, tid 1191, error Operation not permitted

Bug: 208308923
Test: adb logcat | grep SCHED_FIFO

Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
Change-Id: I8142bd33bb67afd53289d23dd1fa3ca54258e08a