fix(magnification): mark a few gesture tests as flaky

Weird that sometimes these flake saying "invalid pointerIndex 0 for
MotionEvent". Maybe the timing is off or something?

Fix: 297879316
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/accessibility/magnification/FullScreenMagnificationGestureHandlerTest.java
Change-Id: Ie92c050fc4e24bb49eb2862a3dc4280bab927b82