[Handoff][3/N] Use RemoteCallbackList for Handoff Callback storage.

Change how HandoffRequestCallbacks are stored to place them in a RemoteCallbackList, which automatically handles if the receiver dies.

Test: Updated unit tests
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: I41f5ca0b8c90df855ec8e79b3e494c307439d1c8