Add type string to VCN NetworkAgentConfig

This change populates the VCN type string used in NetworkInfo. While
the NetworkInfo API has long been deprecated, some apps apparently still
use this API. Even more unusually, some apps use the NetworkInfo's type
strings, and perform string comparisons on the type string, as opposed
to the type integer/enums to determine network types.

This change ensures maximum backward compatibility.

Bug: 219287843
Original-Change: android-review.googlesource.com/1987026
Test: atest FrameworksVcnTests
Change-Id: I059dbc997f97482f7355fdc29d464d5452933ce3
Merged-In: I059dbc997f97482f7355fdc29d464d5452933ce3