Refactors PipPositioner into subclass CollisionDetectionUtils.

Now that PIP is not the only window needing to detect collisions with
other windows, refactors generic collision detection logic from
the PipPositioner.

See go/chromeos-dwell-menu-design, Collision Avoidance section for
design.

Bug: 943703
Change-Id: Idf50f81b7254f22b627858347d1f170d8dccf326
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572860
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: Mitsuru Oshima (Slow 4/22-26) <oshima@chromium.org>
Reviewed-by: Eliot Courtney <edcourtney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653700}
14 files changed