Hotword: Add onStopDetection() for software detectors.

For DSP-based detection, stopRecognition() disables detection at the
sound trigger layer. For software detection, the HotwordDetectionService
must be notified so it can stop detection.

Bug: 189236104
CTS-Coverage-Bug: 183425641
Test: atest CtsVoiceInteractionTestCases
Change-Id: I220d2255e5e500c22723f6e2fbc34860a529f216