AudioFlinger: update primaryPlaybackThread_l() getter

Due to mutex reordering (Hardware mutex moved after effect mutexes),
hw mutex may not be held when acquiring thread mutex.

Bug: 329395147
Test: manual
Flag: EXEMPT bugfix
Change-Id: I1d03d5fa4fe870d9f0bc5ffcc4a2723c709af09d
Signed-off-by: François Gaffie <francois.gaffie@ampere.cars>