Keep STH state in sync with HAL

During stopAllRecognitions(), the internal state of a sound model was
being cleared (which made it look like the model was being unloaded).
However, the model was still loaded, so subsequent calls to load a 'new'
model would break.

Bug:28432002

Change-Id: I7090bf52704c6e46e3bb6d495d8fe4b8a1d9e2ad