[SB Refactor] Provide a disabled wifi repo if there's no wifi manager.

This allows WifiRepositoryImpl to use a non-null WifiManager, which
removes a lot of null checks throughout the pipeline.

Bug: 238425913
Test: verify wifi icon still works
Test: verify wifi demo mode still works
Test: verify that, when WifiManager is null, the wifi icon never shows
Test: all tests in statusbar.pipeline.wifi
Change-Id: I8de76ad554769e91f1612ddfb3e9fa6dfa9bc6ed