Make animators more robust against ending mid-stream

The logic in the frame processing code of ValueAnimator did not handle
the situation of animators being ended while the current animation list
was being processed. In particular, if a call to an animation update
(which could result in a call out to user code) caused that animation, or
other current animators, to end, then there was the risk of running off the
end of the list of current animators.

The fix is to work from a copy of the current animator list, processing frames
on each one only if they also exist in the real animations list.

Issue #6992223 Frequent System UI crash

Change-Id: I742964558f8354f04c311b7b51c7686f26a4dacf