Refactor ExternalCaptureStateTracker

Extract an interface out of it and support registration of multiple
listeners. This enables dependency injection for the tracker, which is
needed by subsequent changes.

Test: atest SoundTriggerMiddlewareImplTest SoundHw2CompatTest
      Manual verification of basic soundtrigger flows.
Bug: 178722883
Change-Id: I146208906391601b7c244c403e0a05879af120ab