Isolate ScreenScaffold insets to prevent status bar padding shifts

ScreenScaffold previously used the global status bar visibility state to calculate its top insets. This caused background screen padding to collapse when opening an overlay or dialog with StatusBarMode.Disabled, and also led to a 1-frame padding jump when mounting disabled screens.
This change updates ScreenScaffold to resolve status bar visibility on a per-screen basis rather than relying on global top-of-stack state.
Relnote: "Fixed an issue where ScreenScaffold top padding collapsed when opening screens with a disabled status bar mode."
Bug: 532112145
Test: StatusBarTest, ScaffoldTest
Change-Id: I7cf4f3b14fbc86ed133e7117ccc60d756522bdc0