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