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
diff --git a/tests/net/java/com/android/server/connectivity/VpnTest.java b/tests/net/java/com/android/server/connectivity/VpnTest.java
index de1c575..565f11c 100644
--- a/tests/net/java/com/android/server/connectivity/VpnTest.java
+++ b/tests/net/java/com/android/server/connectivity/VpnTest.java
@@ -1090,7 +1090,7 @@
                     new String[] { EGRESS_IFACE, "l2tp", expectedAddr, "1701", profile.l2tpSecret,
                             "name", profile.username, "password", profile.password,
                             "linkname", "vpn", "refuse-eap", "nodefaultroute", "usepeerdns",
-                            "idle", "1800", "mtu", "1400", "mru", "1400" },
+                            "idle", "1800", "mtu", "1270", "mru", "1270" },
                     deps.mtpdArgs.get(10, TimeUnit.SECONDS));
             // Now wait for the runner to be ready before testing for the route.
             legacyRunnerReady.block(10_000);