Fix infinite loop problem in AnimationHandler

AnimationHandler.doAnimationFrame() iterates on a list that may be changed
during the course of the lopp, especially when Battery Saver is enabled, or
animators have zero duration, coupled with user code launching new animators
when old ones end.

The fix is to iterate through the old size of the list, and avoid processing
new animators in the current frame.

Bug: 37376806  Keyboard doesn't come up when battery saver is turned on on Android O
Test: manual
Change-Id: I40ca1d849e6f337779c23edbab22d80760c22f9b