Improve test isolation in AbstractLauncherUiTests

- Removing suspicious duplicate uses of AbstractLauncherUiTest.initialize to prevent unnecesary launcher restarts
- Adding UiDevice#pressHome to AbstractLauncherUiTests.verifyLauncherState to ensure the next test starts with a known clean state

Flag: EXEMPT test fix
Fixes: 372956489
Test: AbstractLauncherUiTest
Change-Id: Id68ece4ab195a4f6c47aa401eb50a91b8ff70e10