Thread-safety improvements

Clarify the thread-safe conventions inside SoundTriggerMiddleware and
align code to them. This results in a simpler and more consistent
model, avoiding the need for using atomics in some special cases, etc.

Test: Manual verification of basic soundtrigger flows.
Bug: 178722883
Change-Id: I6ff1cce5d8a3eda261aa732b870d627190855ecf