Allow Listeners to subscribe to RemoteTaskStore

Add APIs on RemoteTaskStore enabling listeners to directly subscribe
to change events, and updated underlying classes (like RemoteDeviceTaskList) to support this. Updated API stubs in TaskContinuityManagerService to pass listeners to RemoteTaskStore

Test: Added unit tests
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: I5ffd0a0819b412590bfa337fc689dfa413a0bf71