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For all event devices, we'll use the device path to find out if it's
permanently attached. Expose this property on EventConverterEvdev and
use it when populating DeviceDataManager.

TEST=chrome on link_freon & rush_ryu
BUG=437539

Review URL: https://codereview.chromium.org/790153005

Cr-Commit-Position: refs/heads/master@{#308151}