Fix ConcurrentModificationException in GenericGestureDetector.

Fixes: 352913191
Flag: EXEMPT bugfix
Test: start ongoing call, then open fullscreen app -> verify status bar
initially appears, but you can swipe up to dismiss it
Test: atest GenericGestureDetectorTest

Change-Id: I09a671c98d68ee256604d35d0f8248439db5903e