Add Stubs for TaskContinuityManager

Create stubbed APIs for TaskContinuityManager, per the design document. Implementation to follow.

Test: Filed API Coverage Bug (listed below)
Bug: 400970610
API-Coverage-Bug: 419601871
Flag: android.companion.enable_task_continuity
Change-Id: I3994fc6608b831f01f2032b2ede785d2bd6017ef