Allow network providers to set yield to bad wifi

While the "yield to bad wifi" policy is handled by CS for
backward compatibility, the network provider should in
fact be in control of it. There is no downside to allowing
providers to set it as no provider is using this, and it
will allow for migration in the future. The code is also
simpler, and becomes possible to test through CTS.

Test: NetworkScoreTest
Bug: 184037351
Change-Id: I3b2418f37bf1f0374d0a626fda7e2b6f3a3b41ba