[sb] StatusBarInitializer to CoreStartable

Decouple the creation of the StatusBarInitializer and subsequent
starting of the CollapsedStatusBarFragment from CentralSurfacesImpl.

Test: atest SystemUITests with the flag on
Flag: com.android.systemui.status_bar_simple_fragment
Bug: 364360986
Change-Id: I9e8043a6105657bc6ac33b25d24a95b900594d6d