[CS] Allow network factory implementations to re-evaluate requests

Change the evalRequests() API to protected to allow network factory
implementations to call the API.

A use-case of the API change is for factories temporarily reject a
request since they cannot fullfill it immediately but retry it at a
later time if/when conditions change.

Bug: 63866251
Test: integration tests
Change-Id: Idc50ce0799c67634b7b2fdbad78a26e443caf2e4