Add API Stubs for RequestHandoff method

This change adds API stubs for requesting a handoff, per the spec outlined in go/handoff-design. Future CLs will implement this functionality and add CTS tests.

Flag: android.companion.enable_task_continuity
Test: Filed API Coverage Bug (listed below)
Bug: 400970610
API-Coverage-Bug: 419601871
Change-Id: I98d590c46e5a4fa8648e6cf335fcd9f1a95afcc5