Fix two flexibility issues

[hqin-work]

1. LOKOverlayLayoutBuilder's "flexibility" block property doesn't have any effect. To fix it, we pass privateFlexibility to LOKOverlayLayout constructor.
2. LOKInsetLayoutBuilder's "flexibility" block property doesn't have any effect because InsetLayout always takes its sublayout's flexibility as its own flexibility. To fix it, we have InsetLayout constructor take in a flexibility parameter and only use its sublayout's flexibility when that parameter is nil.