Support preemptive model unloading

This change plumbs soundtrigger preemptive model unloading, typically
initiated by the STHAL, all the way to SoundTriggerService, where
logic for retry / lazy loading already exists.

Test: Manual verification of existing soundtrigger use-cases.
Change-Id: I49195fbf2ccd6268e7fbc0279ec6fe04c1cc016e