Add base test classes for SplitScreen scenario

All these do is execute the scenario as a test.

They can then be inherited and used with different runners like parameterized runners, FlickerService runners and Microbenchmark Runners.

Test: Bug: 287185020
Test: atest WMShellFlickerServiceTests
Change-Id: I1b57afbffef40ba14abb05ae3bfb28725af2fe2e