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This CL will reland / revert revert 2 commits and add a non-Mash guard
for listening to DeviceDataManager. A TODO has been added about using
InputDeviceClient in Mash, but it's not super urgent as the reaction to
the event is a NOP in Mash further down the call chain.

Revert "Revert of Remove confusing keyboard test & focus on input device"
  commit 8a2f3aa1

Revert "Revert "Listen to changes to touch input devices""
  commit 0cd134a5

Bug: 738885
Change-Id: Ic805ca3966df2969d8444b2c441eac24f75dce62
Reviewed-on: https://chromium-review.googlesource.com/575138
Commit-Queue: Felix Ekblom <felixe@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488622}