shill: cellular: Use the "Broadband" APN by default for AT&T.

The "Broadband" APN should now be used by laptops for data connection. This CL
changes the order of AT&T APNs, so that we hit it on the first try.

BUG=chromium:372076
TEST=Manually connect to AT&T with different modems. Verify that we connect to
     "Broadband".

Change-Id: Ice3dd84340e5baae0eb5451379f9c2f9740a3fe1
Reviewed-on: https://chromium-review.googlesource.com/199230
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>