[NS04] Introduce Network Offers and their callbacks

This patch introduces the concept of a network offer that
providers send to Connectivity to register for relevant
requests. This lets them see only requests that they can
hope to satisfy considering their capabilities and score
filters.

This is meant to replace the filtering mechanism currently
implemented by NetworkFactory. The reason for replacing
this mechanism is that the old mechanism does caps and
score filtering on the factory side, which requires these
two filters to be contextless and available system-wide,
including in separate processes from the system server.
These constraints severely limit and complexify in
particular what the score comparisons may look like. In
the past the score comparison was only integer-based,
making the code duplication not much of a problem, but as
this scheme is becoming unsustainable by spreading the
complexity of the selection across the entire stack, a
centralized mechanism is now necessary.

This patch only introduces the new objects and has CS
keep track of them, but does not actually use them yet.
Followup patches will implement the logic of calling
the offer callbacks.

Test: FrameworksNetTests NetworkStackTests FrameworksWifiTests
Bug: 167544279
Change-Id: Idec1fe8eb4ac6f562bf098e3dd470f11024d04f2