Set TRANSPORT_PRIMARY for VCN NetworkOffers and NetworkAgents

This change sets TRANSPORT_PRIMARY for all NetworkScore objects passed
to ConnectivityService by the VCN. Without this patch, a VCN in safe
mode will never get any NetworkRequests, since the new Policy-based
NetworkRanker always prioritizes a existing satisfier of a network,
regardless of score. However, a higher-priority policy check ensures
that a TRANSPORT_PRIMARY request is used if it exists. This changes the
VCN to utilize that functionality.

Bug: 194122634
Test: atest FrameworksVcnTests
Original-Change: https://android-review.googlesource.com/1772025
Merged-In: I3a298a6c4c594c6e224f3f05764614802c00e9b0
Change-Id: I3a298a6c4c594c6e224f3f05764614802c00e9b0