1/ Migrate cts SplitScreenTests to wm shell flicker test

This is the initial version to achieve:
1. SplitScreenActivity in test-apps
2. Create SplitScreenTestBase and add a basic test cases to
   ensure SplitScreenActivity enter split screen mode
3. Assert StatusBar/NavBar/SplitScreenActivity showing when
   device enter split screen mode
4. Verify docked apps ending bounds

Bug: 169271943
Bug: 172811376
Test: atest WMShellFlickerTests:EnterSplitScreenTest
Change-Id: I26461b958c578dcbcc5ca2235f9d269559fd9ec5