Add tests verifying task snapshot is taken when entering PiP

This change adds two unit tests to verify that `TaskSnapshotController.recordSnapshot()` is called when an activity is moved to the pinned (Picture-in-Picture) root task.

The tests cover two scenarios:
1.  A task with a single activity entering PiP.
2.  A task with multiple activities, where the top activity enters PiP.

These tests confirm that a snapshot of the correct task is taken in both cases, ensuring a valid thumbnail is available for display in places like the Overview screen after exiting PiP.


Please help fill out the survey for feedback: https://docs.google.com/forms/d/e/1FAIpQLSeKFKpHImCAqZIa_OR801cw72HQUreM2oGM25C3mKKT2tBFnw/viewform?usp=pp_url&entry.1586624956=ag/35406354

Please feel free to use your domain knowledge to make changes to the generated tests to make it more valuable for your team.

Original Change: ag/34943972 (Note tests are based on the original CL but may add coverage beyond the specific changes to improve overall code health)

Test: ATP tests passed http://go/forrest-run/L97000030017377051
Bug: 431235865
Flag: TEST_ONLY



Change-Id: If4961ff1b96a723c83810bdf2ccdfb14ed7a5fbb