Fix non-resizable split flicker tests

1. Update the flag of DEVELOPMENT_ENABLE_NON_RESIZABLE_MULTI_WINDOW
during tests.

2. Before, the tests only checked whether the app window/layer became
visible/invisible, which could also happen during opening recent. Now,
we also check the visibility at the end.

Bug: 182235458
Test: atest WMShellFlickerTests
Change-Id: Ibc7b359d600a0e4105cc3a84b3111d59813eb9f1