audio: Update ChildInterface to set scheduler policy lazily

Since the client may end up never retrieving the interface
instance for sending it over Binder, postpone setting
of the scheduler policy to until that very moment.

Rename 'ChildInterface::getPtr' to 'getInstance' for clarity.

Bug: 264712385
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I31f9dd832c7f85f9632d1d389a8b1063d237d4c1