fix(window magnification): mark test as flaky

Nothing really we can fix about spy() getting confused about which
method to call. Ideally Mockito would fix this since it seems like
a pretty severe problem.

Fix: 297879435
Test: atest services/tests/servicestests/src/com/android/server/accessibility/magnification/WindowMagnificationManagerTest.java
Change-Id: I6cbb37e0cb68cceb533a63ed73c3ce520ab5c186