audio: Start Binder pool thread explicitly

It was noted that calling 'ABinderProcess_startThreadPool'
results in the audio HAL service having more Binder pool
threads than without it. This is preferred to having
just one thread via 'ABinderProcess_joinThreadPool'
as in the latter case clients may seemingly be blocked
instead of being served by a newly spawned thread from
the pool.

Bug: 357669241
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Ia8ed52b3c5a95cea12d063a0e75ac908af9260f8