Reduce redundant animation processing

Starting several animations will place separate events onto the
animation queue, which may cause the active animations to get
processed more than once in any frame when one of those start messages
is processed.

This change moves the logic of starting pending animations into
the animation frame processing itself. Now when a start event is
processed, it only calls the animation frame logic if there are
unstarted animations pending.

Issue #6172602 Inconsistent animation callbacks

Change-Id: I3a546f0c849f42b2dd998f099fcdfafd7d780ad9