Add tests for hiding split-screen divider during launch

This change adds unit tests to verify the behavior of hiding the split-screen divider when launching tasks into split-screen under specific conditions.

The tests cover the following scenarios:
- Hiding the divider when a stage is inactive.
- Hiding the divider when split-screen is not visible.
- Not hiding the divider when stages are active and split-screen is visible.
- Ensuring the divider is not hidden when the feature flag is disabled.



Please help fill out the survey for feedback: https://docs.google.com/forms/d/e/1FAIpQLSeKFKpHImCAqZIa_OR801cw72HQUreM2oGM25C3mKKT2tBFnw/viewform?usp=pp_url&entry.1586624956=ag/35415960

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/34862688
Test: ATP tests passed http://go/forrest-run/L76400030017434709
Bug: 431235865
Flag: TEST_ONLY



Change-Id: If14b58706428818a191cba47655c8047ebd0bb39