[Handoff][3/N] Return error if requesting Handoff from an unknown association.

This is part of a series of changes adding Handoff to AOSP.

This change blocks issuing Handoff requests to associations that are not currently connected, instead returning an error to the caller.

Test: Added Unit Tests
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: Ib2e6e7e0bd396fae5c6132e498c658648dddb050