cellular_OutOfCreditsSubscriptionState: Use Plugin property to emulate modem

Shill now looks at the ModemManager plugin property to determine which
OOC detector to use, update test accordingly.

BUG=chromium:395960
TEST=cellular_OutOfCreditsSubscriptionState

Change-Id: Ib983c3484048ec1c457ee13a5a80de30b79b3e3c
Reviewed-on: https://chromium-review.googlesource.com/209697
Reviewed-by: Ben Chan <benchan@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Thieu Le <thieule@chromium.org>
2 files changed