us: add "" APN for T-Mobile

The "" APN accommodates LTE plus what the
"" APN already supports.

TEST=Tested the following:
1. `xmllint --valid --noout serviceproviders.xml`
2. Run unit tests.
3. Connect to T-Mobile using Gobi2k, Gobi3k, Y3300, Y3400, and MU736.

Change-Id: Ie8946939206c7057f6d1b6da2e489f746bdd938d
Reviewed-by: Thieu Le <>
Commit-Queue: Ben Chan <>
Tested-by: Ben Chan <>
1 file changed