EXO: Refactor zcr_vsync_feedback interface

Move the zcr_vsync_feedback interface into its own files. This change
only moves code around, no functional changes.

Bug: 896710
Test: Build and run exo_unittests
Change-Id: I74885b5a6692306b05a1d86350f627ed7046045b
Reviewed-on: https://chromium-review.googlesource.com/c/1427571
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625190}
4 files changed