Mark the test as flaky, to unblock presubmit.

Turns out class inheritance doesn't work well with annotations, they are not being overriden.

Test: n/a, since it's annotations, but also: atest WMShellFlickerTests:AutoEnterPipOnGoToHomeTest
Bug: 238367575
Bug: 238255886
Change-Id: I337742b75643e633e4bc0e2815da1eab19c4441c