Fix race condition

 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