Remove VpnType usage in VpnTransportInfo

The VpnType annotation is a hidden symbol, and should be
kept hidden as annotations are disallowed by API guidelines.

Remove its usage in VpnTransportInfo as users of annotated constants
that build against API stubs are expected not to use the annotation.

Bug: 173331190
Test: m
Change-Id: I171fa57f6279defad081c3cd16265d58ec55e57d