Remove MessageUtils usage in VpnTransportInfo

MessageUtils is a hidden utility, and including a jarjared copy in
framework-connectivity would add complexity.
It is only used in VpnTransportInfo, where it would parse VPN constants
when the class is loaded in each process. Considering the performance
and maintenance cost using numerical type codes in toString() seems to
be a better tradeoff.

Bug: 177046265
Test: m
Change-Id: Ie71cc816f86e020b44ed1c86349b5c9204dee3cf