Add a regression test to verify no IME flickering when navigating back

As CL[1] changes the semantic of WindowState#isVisibleOrAdding that
affects computeImeTarget result when finishing the activity with IME has
shown by back navigation, that caused IME flicking after backing
to the previous app.

Add a flicker test to verify the above test scenerio won't happen.

[1]: Ic537d77c8b0e10e80444dd46d7b49e5a9bd63d6c

Bug: 205075200
Bug: 205761007
Bug: 204948120
Bug: 204731658

Test: atest OpenImeWindowAndCloseTest
Change-Id: Ibe582ccd73aa3b2326040f5d2e6e5edd9de95605