shill: make property identifier for Cellular.ModelID consistent

This doesn't follow the style guide, so add another identifier that
does and a note to remove the other one later.

BUG=None
TEST=emerge

Change-Id: Ia772121774236ee056ab0961e0e000f16c1476ce
Reviewed-on: https://chromium-review.googlesource.com/738457


Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>