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Exo previously dispatched all touch events to one focused surface: the
surface where the first touch point landed. For ARC++, this does not work
for supporting multiple displays because the display for the touch is
determined from the surface of the touch event. Therefore each touch
down event needs to be sent to the correct surface that it was directed
to.

This CL adds two unordered_maps to track the touch pointers and their
target surfaces: one mapping the touch pointer id to its surface, and
mapping the surface to the number of pointers on the surface. The latter
is needed to know when to stop tracking the surface's lifecycle.

Bug: b/145027838
Change-Id: I4121dac9c39bd9fb569df94330107ed382fe7234
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1931584
Commit-Queue: Prabir Pradhan <prabirmsp@chromium.org>
Auto-Submit: Prabir Pradhan <prabirmsp@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732700}