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In preparation for reusing touch transformations for transforming on
screen stylus events into display co-ordinates this CL does a simple
refactor of TouchTransformerController.

This change will be followed up by a subsequent CL that enables support
for having multiple touchscreens per display with the per-device looping
happening in the new methods: UpdateTouchRadius() and
UpdateTouchTransform().

TEST=ash_unittests passes, interactive testing on link_freon in mirrored
and non-mirrored mode.
BUG=448467

Review URL: https://codereview.chromium.org/1130983005

Cr-Commit-Position: refs/heads/master@{#329830}