Move interfaces out of NetworkController

This CL moves some interface and class definitions out of
NetworkController. AccessPointController and SignalCallback get their
own files, and MobileDataIndicators and WifiIndicators both moved to the
SignalCallback kt file.

This doesn't simplify things just yet, but it does clean up imports and
interfaces, and moving the *Indicators classes to kotlin gives us the
data class niceties.

Test: atest SystemUITests
Bug: 197851948
Change-Id: I6eba271cdc28d81bc1d281eab2e6c230f4ad89cb