Switch to using NetworkScore instead of legacy integer

This change switches the VcnNetworkAgent from using the legacy score
integer, and switches to using the new NetworkScore, using the legacy
integer as a internal representation.

Bug: 185204197
Test: FrameworksVcnTests
Merged-In: Ib0690243f176a2f074358fcc8430f1fd7c3d0d3c
Change-Id: Ib0690243f176a2f074358fcc8430f1fd7c3d0d3c
(cherry picked from commit ffc912da)