Make VpnProfile.maxMtu default value match Ikev2VpnProfile

This change corrects the VpnProfile's maxMtu defaults to match that of
the Ikev2VpnProfile. 1400 is too high as a default, and Settings will
run into an issue here quite often.

Bug: 152573931
Test: FrameworksNetTests passing
Change-Id: I97ba5903b3cc1ed6a21c706ed3d78bd8ecbeee0c

tests/net/java/com/android/internal/net/VpnProfileTest.java

@@ -65,7 +65,7 @@ public class VpnProfileTest {
         assertTrue(p.getAllowedAlgorithms() != null && p.getAllowedAlgorithms().isEmpty());
         assertFalse(p.isBypassable);
         assertFalse(p.isMetered);
-        assertEquals(1400, p.maxMtu);
+        assertEquals(1360, p.maxMtu);
         assertFalse(p.areAuthParamsInline);
     }