[Handoff][8/N] Store HandoffActivityData on ActivityRecord

When an `Activity` is stopped, persist HandoffActivityData if available on the ActivityRecord - allowing for Handoff to occur in the background.

Flag: android.companion.enable_task_continuity
Bug: 400970610
Test: Updated Unit Tests
Change-Id: I1164d328f74fbc38cb00dbe0428798974596f986