Add ServiceHolder for AudioPolicy

Subsequent commits require registering tasks on the audioserver
lifecycle.

Pull out management of the binder service lifecycle into a holder class
which can register tasks on start/death. Modify DefaultAudioPolicyFacade
to use the new ServiceHolder.

Bug: 338089555
Test: atest ServiceHolderTest
Test: atest AudioManagerTest before and after kill audioserver
Flag: EXEMPT: mechanical refactoring
Change-Id: I2a72b0800d5073d3007bf46561ffc857fc43fe47