[NS01] Add NetworkScore

As attested by numerous TODOs in the code, a new way of
representing network quality and policy is needed instead
of an int.

An int representing the quality of the network requires
all parties using it to know how all other parties are
using it, and implementation details about the decision
algorithm. For all intents and purposes, the selection
is left to individual network factories who try to
achieve a desired result while piecing together all
possible states of the system.

As the number of such cases and desires increases, this
becomes both intractable and unmaintainable. Indeed, at
this time in the codebase nobody can really predict exactly
how a given change in score will affect selection across
the board, and it is essentially impossible to figure out
the behavior of network selection by inspecting the code
because the moving parts are scattered throughout the
entire codebase.

Having an object encapsulating policy and quality values
will let us centralize the selection and make it again
possible to maintain without knowledge of all behaviors
of all network factories. It will also provide better
guarantees of respecting policy, and allow bugfixes that
were not possible before because they'd touch too many
parts of the code.

Test: FrameworksNetTests FrameworksWifiTests NetworkStackTests
Change-Id: I3185a6412b9b659798faf0c6882699e9c63cc115