audioflinger: Adjust priority of hal threads

HALs are prohibited from using framework binder, and there is
no equivalent scheduling policy service in hwbinder. Thus, in order
to match priorities of FastCapture / Mixer threads with their
counterparts in the HAL, it is needed to request the priority boost
from audioflinger on behalf of the HAL.

Test done to verify the priority was correctly set.

Bug: 34131400
Change-Id: If8b6b031c0fcba771fae901a5b8e7da89b3a1570
Test: check priority match between audioflinger's and hal's threads