Mark WindowMagnificationAnimationControllerTest as flaky

The WindowMagnificationAnimationControllerTest is flaky on presubmit, but the tests are passing both on local atest and the remote testing system. We would like to mark the test as flaky to avoid blocking in the test monitor dashboard and would like to remove the filter after checking another animation test related issue.

Bug: 308501761
Flag: N/A
Test: N/A
Change-Id: I4f6f0d432cf29e2a9fbe83632aea9632abdef9f0