Mark OpenAppToSplitScreenTest focus test as flaky

From the logs, sometimes recents_animation_input_consumer
does not get focus. Disabling until we have a change to
investigate further.

Test: atest FlickerTests:OpenAppToSplitScreenTest
Bug: 151179149
Change-Id: I8e15f074a58fecf4f7059a4cd1d0135a98cc9770