Send a tryAgain() when no longer at model capacity

This makes sure a tryAgain event is sent whenever we unloaded a model
and as a result we're no longer at our max model capacity.

This change also gets rid of atomics in favor of common locking.

Test: atest SoundTriggerMiddlewareImplTest SoundHw2CompatTest
      Manual verification of basic soundtrigger flows.
Bug: 178722883
Change-Id: I053fd23802de93a416930dd2456c91d1a6bb91fb