Fixed LayoutTransition bug moving multiple views

The problem was that there can be >1 animation spawned for each
view in a container, if there are multiple events that trigger
a transition. These animations would potentially clobber object
layout values, causing problems as successive animations tried to use those
clobbered values to set up their own animation values.

The fix is to track the created animations and cancel them as future
animations on those same objects get created. This mechanism used to
be in the code (the bug came about when that mechanism went away), but
was removed because of memory leaks of never removing animations that
were set up but never started. The new approach also caches pending
animations, but runs a second aniamtor to delete the entries in that
collection just in case.

Change-Id: If60c7d188712334dea69d0794dc6b4ce29ca6c09