Expose NetworkAgent constructor taking a NetworkScore parameter

Vpn constructs NetworkAgent with this exposed NetworkAgent
constructor. Given NetworkAgent is moving into the incoming
connectivity module, Vpn which is outside the module will not
be able to access it if it's not in the API surface. Thus,
expose it to allow Vpn to use it.

Bug: 182963397
Test: make update-api
Change-Id: Ic2357dcfff3233e0dd17b48b0e376a5095ef60fa
CTS-Coverage-Bug: 172183305