Annotate entireScreenCovered in split to pip test

Annotate FromSplitScreenEnterPipOnUserLeaveHintTest#entireScreenCovered
as flaky while we investigating the case.

Seem like that, half of the display where the will-pip app was becomes
empty at the beginning of the transition.

Bug: 336510055
Test: atest WMShellFlickerTestsPip1
Change-Id: I36ed7c3b41f21dd8d6a76601e471d4e5551923ae