libaudiohal: Remove unused protected methods from StreamHalAidl

getHalPid and requestHalThreadPriority are not used
because in the AIDL model it's the responsibility of
the HAL service to set the priority for its threads.

Bug: 205884982
Test: m
Change-Id: I5278f544eadc184d109e3b396cc72b70a47d5557