Add tests for split button visibility on external displays

Adds unit tests to verify the visibility of the split-screen button in the handle menu when a task is on an external (non-default) display.

The tests cover the behavior with the `enable_non_default_display_split_bugfix` flag both enabled and disabled, ensuring the button is shown or hidden correctly according to the flag's state.

Please help fill out the survey for feedback: https://docs.google.com/forms/d/e/1FAIpQLSeKFKpHImCAqZIa_OR801cw72HQUreM2oGM25C3mKKT2tBFnw/viewform?usp=pp_url&entry.1586624956=ag/35484447

Please feel free to make changes to the generated tests based on your domain knowledge. More context about the project please see go/ai-testgen

Original Change: ag/34622022 (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/L90900030017612015
Bug: 431235865
Flag: TEST_ONLY

Change-Id: I7a7d0db246973b6e5177fcfa13efac7f0bc43d94