End staging animators when destroy RootRenderNode

When animation happens in a dialog, it is possible for the dialog to
be dismissed and RootRenderNode to be destroyed before we create
animation handle for the staged animators. In that case, we need
to remove the staged animators so they will not run without a
animation handle.

Bug: 26975079
Change-Id: I0c2c6c1b530beaec3984c0b1c410df4fd8f25c95