Change test to check for primary app bounds instead of layer visibility

* App layer will now always be visible since the layer above it
is visible with corner radius change
* Bounds are a more accurate representation of visibility. If we want
to ensure that the top layer is actually present, we should have another
test to check that the top layer isTranslucent=true

Bug: 436897465
Test: atest WMShellFlickerTestsSplitScreen-DismissSplitScreenByDividerTest
Flag: EXEMPT test
Change-Id: I7469c4ccc919e3bbb5344de42567c4ce43024270