Multi display initialization of Status Bar

Minimum required changes to initialize and attach the Status Bar on
multiple displays.

This part is just about initializing and attaching. Things like
per display icon colors, bar modes (and many other things) are still
not implemented.

Test: MultiDisplayStatusBarStarterTest
Test: Manually - Add a virtual secondary display and verify status bar
      shows.
Fixes: 367592591
Flag: com.android.systemui.status_bar_connected_displays
Change-Id: I7da71840e550592c9fd96c707210444e9cf3d220