SoundTriggerHelper: fix power save mode listener

When stopping a recognition, do not unregister power save mode
listener if no recognition is active any more but only if no recognition
is requested. Otherwise recognitions suspended due to power save mode will
never resume.

Bug: 136978230
Test: Activate power save mode, plug USB and verify OK G works.
Change-Id: I426e11447fcde5c578ae2f64aa1fe18ef00fba0f