Pull HandoffActivityData from Running Activities

This change adds a new internal API on ActivityTaskManagerService, allowing services to request HandoffActivityData for a task. For now, this API will only pull HandoffActivityData from the topmost Activity while it is running. Future CLs in this stack will enhance this functionality.

Test: Added unit tests for ActivityTaskManagerService. Due to the structure of ActivityThread, unit testing isn't really possible. The full flow will be tested in CTS tests once wired up fully.
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: I555b7d55b0f1d1048b52bbfe06a7793b2a13948b