Rename TapProducer to TapSubscription.
Rename TapProducer to TapSubscription.

Summary: Avoiding introducing new names where possible. TapSubscription also more accurately describes what the object is (a subscription to tap events).

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

Reviewed By: #material_motion, O4 Material Apple platform reviewers, O2 Material Motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D2184

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