Speculatively fix L2TP keepalive issues by reducing mtu

This change reduces MTU/MRU values for racoon/pppd from 1400 to 1388 in
an attempt to fix keepalive issues experienced in the field.

Bug: 119076193
Test: Speculative change, original issues were not reproducible.
Change-Id: I7731503260fdc8bcbc6c5fcda6991ead6fb7c52e