[speech] implement finalizer for SpeechRecognizer objects

This change is needed because GC'd SpeechRecognizer objects will leak an open binder connection and cause a leak in system_server until this process dies. This finalizer ensures we call destroy when SR is garbage collected, which notifies system_server to do internal cleanup.

Test: atest CtsVoiceRecognitionTestCases
Bug: 297249772

Change-Id: I04061a84e21fab3ffbabc68d003c4436a27f837f