Invert how sound trigger capture is notified

Previously, audio policy client would call an method on the sound
trigger middleware service. This has been replaced by the sound
trigger service registering a callback, in order to avoid a cyclic
dependency between the services.

Bug: 146157104
Merged-In: I876461a7650d337d6dd05fbe80efaf08f8ef20dd
Change-Id: Ia8f674d7dd50f6dcdd6595bcbadec2f25fdb3561