[Handoff][3/N] Serialize Handoff Enablement in RunningTaskFetcher

This change updates `RunningTaskFetcher` to include if Handoff is enabled in a task

Flag: android.companion.enable_task_continuity
Bug: 400970610
Test: Added Unit Tests
Change-Id: I1a314d2dfd3f81bd130b2252f1e42b80f9eb65fc