[NS B04] Make the network selection request-major.

This patch marks the most important turning point of this
refactoring. Cleanups removing unused code will follow.

Replace the old network-major reassignment computation with a
much simpler and faster loop that takes each request and assigns
it the highest-scoring network.

All tests pass, of course.

Bug: 113554781
Test: FrameworksNetTests
Change-Id: Ie143802995155151a38a4eb1d2f26c3f29e556bd