Configure MTU based on network MTU parameter

Add logic to obtain the mtu from the network PCO parameter and set it to kernel
when the mobile data connection is established. When there is no PCO mtu configured
from the network, the mtu size defined in the corresponding APN will be used. In case
no mtu size is defined for an APN used for data connection, the MCC/MNC based MTU
defined  in the framework overaly will be applied.

bug:17046179
Change-Id: I6465d4b8f2076aaa380ae3617fb3f24adbe136d4