SoundTriggerHelper: Remove unused models onLoad

If some process tries to start a recognition, the STH will now check to
see if there are any other models that have been loaded and either:
 - Have a callback that's died (determined by pinging the binder)
 - Don't have a callback at all (meaning they didn't clean up after the
   last detection and didn't start it again).
If it finds such a model, it will be stopped and unloaded, freeing up
resources in the HAL.

Test: Manually load model/kill the process and make sure that it is
unloaded the next time some other model is loaded. Also made sure
running models aren't evicted.

Change-Id: Ia1139b47afe5f88102a3773d90f4f056ea3cdc92