SystemUI Carrier Network Change unit tests

- Includes unit tests for verifying mobile data indicators.
- Found one bug where dark mode icon wasn't showing properly when
  different from light mode icon.
- Comment out failing test
  NetworkControllerSignalTest#testSetCurrentSubscriptions

Bug: 20288155
Change-Id: Ib3c9ba224c4187cab35d6bfa68f6bd4c489cf98e