Setting MTU size for specific network.

Able to config network specific MTU size. Normally, the default size of MTU is 1500.
 US - ATT 1410, TMUS 1440, SPRINT 1422
 KR - SKT 1440, KT 1450, LGU+ 1428
 JP - KDDI 1420, SoftBank 1340
 CA - RGS 1430, FIDO 1430, MTS 1430, BELL 1358, SaskTel 1358
 AU - TEL 1400

Bug: 10195070
Change-Id: Ie18650b37a3d44af944f2dae4aa97c04fb12cd5e