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Currently we stub out TouchEventConverterEvdev::Initialize() in the
tests, but we now need to fix that because that's where we'll detect if a
device is MT-capable.

Initialize() looks up slot values, so we'll need to populate them from
captured device descriptions in CapabilitiesToDeviceInfo(). The initial
state is all zeroes except for tracking id (this matches kernel initial
slot state).

While we're here, clean up a bunch of the slot values setup & access
code in EventDeviceInfo.

BUG=461518
TEST=events_unittests

Review URL: https://codereview.chromium.org/1037843003

Cr-Commit-Position: refs/heads/master@{#322580}