Add test transition ExitSplitScreenFromBottomTest

Add a new test (only the transition) called ExitSplitScreenFromBottomTest. This test will open an app in splitscreen and then it will exit the split screen by moving the divider to the bottom (similar to SplitScreenToLauncherTest that moves it to the top).

Test: atest WMShellFlickerTests
Bug: b/171049505
Change-Id: I42feb423d346829f829404db13590e70bed16f21