Better mapping lookup for demo mode

Move the entire lookup into `MobileMappings` to the
repository layer. So when network types come down, they are immediately
transformed into `ResolvedNetworkType` objects and given a key at that
moment, using the default mobile mapping config

This allows us to A) better insulate us from external depenencies and B)
implement a very trivial mapping for Demo mode. TBD on whether or not we
should allow switching to use the real mobile mappings implementation

Test: tests in tests/src/com/andrdoid/systemui/statusbar/pipeline/mobile/*
Bug: 261185097
Change-Id: I727ad3ca823f3eb225a56f371aa3cef46c340f57