speech: Timely model download listening connection termination

Callbacks onSuccess, onSchedule and onError in ModelDownloadListener
should be terminal ones, i.e., no callbacks should be called
after one of these. Callback forwarding will be stopped
inside the recognition service.

Bug: 283102476
Test: test app, atest CtsVoiceRecognitionTestCases
Change-Id: If1fdb09bd10ae36fe3eb02c562e6d2069745bb8e