Bug: 17372309
AnimationContext::startFrame() happens both with and without
the UI thread lock. Pass the TraversalMode into it so
that ThreadedRenderer's subclass can correctly decide
when it is safe to push over mPendingAnimatingRenderNodes, as doing
so outside of the lock is Very Bad.
Change-Id: Ife5dd3a2b46b0a207cd9234c159a674afdbf5efd