libril: use ip address as gateway in ppp network.

Fix data until these commits are completed:
http://review.cyanogenmod.org/82277
http://review.cyanogenmod.org/82329

Credits to Howard Su
http://review.cyanogenmod.org/80947

Change-Id: Iab88fd8d4ed1089cfdb724aa460597f6e39003bc
2 files changed