API to destroy active detectors

The active AlwaysOnHotwordDetector and SoftwareHotwordDetector is always
maintained  in memory regardless if the client is using it or not.
This added API allows an active VoiceInteractionService to indicate that
they will no longer use the detector, and it can be cleaned up.

Test: atest HotwordDetectionServiceBasicTest
Bug: 193232191
Change-Id: I47c6c64c5c85c01e75ddc6bc504664883a57730b