API for network cache subsystems.

A network subsystem (e.g. Wi-Fi) will be responsible for implementing
INetworkScoreCache. NetworkScoreService will load caches for each
configured network subsystem. When it receives a call to update
scores, it will separate the networks by type and call updateScores on
the appropriate INetworkScoreCache.

Each subsystem will also be responsible for requesting scores for new
networks by calling NetworkScoreManager#requestScores.

Bug: 14408144
Bug: 14137255
Change-Id: I76e5e5205bc359c39d6b9c1782126fb7f465cd96