[sb] status_bar_simple_fragment -> status_bar_root_modernization

This flag rename better reflects the intent of this flag. When this flag
rolls forward, we will have a modernized root view of the status bar.
I.e., CollapsedStatusBarFragment is replaced with an @Compose root

Test: atest SystemUITests
Test: compile
Bug: 364360986
Flag: com.android.systemui.status_bar_root_modernization
Change-Id: I305f6b520a1a9e7e4451846d910a243ada4350c5