SysUI: Refactor the NetworkControllerImpl

Minimal changes to interface/callbacks, all of the changes are
internal and prepare for MSIM.

Separate out AccessPoint and MobileData from the NetworkController
interface to give some space.

A SignalController class has been created as a base for both
WifiSignalController and MobileSignalController, both of which
internally handle the state of their respective connectivity and
only reach up into the NetworkControllerImpl when completely
necessary (such as for combined carrier label).

Bug: 18222975
Change-Id: I75b954bbece187371cdb8571dd8420e7d2cad978