[Handoff][1/N] Add InboundHandoffRequestController

This is part of a series of changes that add the Handoff feature to AOSP.

This change adds a controller to process incoming Handoff requests. Requests are then forwarded to `ActivityTaskManager`, which provides task data to send back to the requesting device.

Test: Added Unit Tests
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: I164adab0e65664a16136efa4e32eaa8e185530bd