Fix flaky uncropped_container_replaces_touchable_region_with_null_crop

The test injects an event that its windows should not receive.
Since this is an integration test, that event can go to other windows
underneath and cause unintended side effects, like changing touch mode.

Create a background container layer where the injected touch will go
to prevent it from going to other windows that can potentially affect
the test.

Bug: 376041354
Change-Id: I252a5ca9c7d4bccd90b85f243e4fba5a1c44b8b7
Test: presubmit
Flag: TEST_ONLY