Add an @Inject-able MobileSignalControllerFactory

This is step (1/n) towards refactoring the networking iconography stack
in sysui. This change moves the creation of the MobileSignalController
to an injectable factory as a precursor to reducing the dependencies
between NetworkControllerImpl and its managed subcontrollers.

Test: atest SystemUITests
Bug: 238425913
Change-Id: I405ad38588a3d904675de7c64142d22e4402f8e5