1/ Refactor LegacySplitScreenTests

1) Separate NonResizable activity tests
2) Fix flaky in visibleWindowsShownMoreThanOneConsecutiveEntry
3) Fix flaky in OpenAppToLegacySplitScreenTest
4) Align implementation of ExitLegacySplitScreenFromBottomTest

Test: atest WMShellFlickerTests
Bug: 176897747
Bug: 176923301
Bug: 176134495
Change-Id: I1e3efdbb1645f6d1a77950101e2c9f5b34856a2f