Fix PiP Flicker test transitions

Ensures we don't have no-op transitions anymore. Another change makes it
so we assert and fail if we have transitions we no entries to make sure
we don't fail silenetly in these cases.

Test: atest FlickerTests WmShellFlickerTests
Bug: 247464790
Change-Id: Iaad739c4bb869c51c6deee582ae5ff9c81013ea2