[Handoff][6/N] Add @NonNull Attributes to RemoteDeviceTaskList

Minor cleanup on RemoteDeviceTaskList - add @NonNull and @Nullable attributes, and check for null on arguments.

Flag: android.companion.enable_task_continuity
Bug: 400970610
Test: Updated Unit Tests
Change-Id: I12729ad0433bbc488c65bdd695c63e8949671291