Extract out rotationLayerAppearsAndVanishes assertions

Fixes problem with rotationLayerAppearsAndVanishes_shellTransit calling rotationLayerAppearsAndVanishes which included the assertion along with an Assume.assumeFalse(isShellTransitionsEnabled) which meant the test would never run

Test: atest FlickerTests:ChangeAppRotationTest
Change-Id: Icf637f09a062e5f56cab4b5b28fbba7208b0fea4