us: add "pta" APN for AT&T

BUG=chromium:306342
TEST=xmllint --valid --noout serviceproviders.xml
TEST=Run unit tests

Change-Id: I46b1a74538fe13f5b1d42ba417a1537a27b58826
Reviewed-on: https://chromium-review.googlesource.com/172702
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Thieu Le <thieule@chromium.org>
1 file changed