Fix deadlock between audio policy manager and sound trigger m/w

Audio policy manager and sound trigger middleware services both call
into each other and each has global locks, which may result in a
deadlock. We previously made sure that calls from sound trigger into
APM are done outside of the lock, but have missed a lock in one of
the decorators, namely SoundTriggerMiddlewareValidation.

Bug: 150843018
Test: Manual verification
Change-Id: I53dfd15e137a5137d664e75e5451c444ab848489