Add test for switching focus between 2 apps.

- Assert that the last launched app is on top at the beginning.
- Assert that newly focused app is on top at the end.

Test: atest SwitchFocusBetweenAppsTest
Fix: 418215569
Flag: EXEMPT adding tests
Change-Id: I1359c2c57a0d8e372b3db0cbac6c21a6fd6da2ed