[AE] Prevent crash when launching activity's task id is invalid

If we cannot get a valid task id for the launching activity, then we
fall back to treat it as a non-activity context. This prevents crash
when creating task fragment with invalid task id.

Bug: 391438117
Test: atest SplitControllerTest  ActivityEmbeddingLaunchTests
ActivityEmbeddingLifecycleTests ActivityEmbeddingPolicyTests
Flag: EXEMPT bugfix
Change-Id: I3209ac784ea5f97cda81a11f908d7eceead347db

Change-Id: I1ee93340ce8d7af776592182cb5240bd62e48736