Always consume bottom insets when navigation bar is force shown

When an app requests SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION but we
force show the navigation bar, we need to treat for the app like
there is no virtual navigation bar on the device. Because if you
combine it with FLAG_HIDE_NAVIGATION, you'd expect the navigation
bar gets hidden but it doesn't, so there could be content that
overlaps with the navigation bar.

Bug: 27157904
Change-Id: I088e02eae2e723c35e9cb4873de6b1325458533b