Updating split screen tests

Updating split screen tests:
- Adding the assertions visibleWindowsShownMoreThanOneConsecutiveEntry and visibleLayersShownMoreThanOneConsecutiveEntry for all the split screen tests (the layer one is disabled as we need ag/13051160 to ignore one layer).
- Replacing some assertions for other that check more things (i.e. replacing the dockedStackDividerIsVisible assertion for dockedStackDividerBecomesVisible).
- Adding some new assertions to the current tests in order to check for more things.

Adding new WM and layer assertions to Flicker tests to avoid repeating code:
- appWindowBecomesVisible
- layerBecomesVisible
- layerBecomesInvisible

Adding new layer assertions to WM shell to avoid repeating code:
- dockedStackDividerBecomesVisible
- dockedStackDividerBecomesInvisible

Test: atest WMShellFlickerTests
Bug: b/171049797
Change-Id: I2bbb23a1cf92df16ee6292e5da4644936a6d99e5