Add MotionAggregator delegate.
Add MotionAggregator delegate.

Summary: This will allow external clients to observe aggregate state changes to a motion aggregator.

Reviewers: chuga, O4 Material Apple platform reviewers, O2 Material Motion, #material_motion

Reviewed By: chuga, O4 Material Apple platform reviewers

Tags: #material_motion

Differential Revision: http://codereview.cc/D2393

material-motion/material-motion-swift,material-motion/material-motion-swift