[Tabs] MDCTabBarViewController should delegate status bar to children (#2126)
[Tabs] MDCTabBarViewController should delegate status bar to children (#2126)

Because MDCTabBarViewController displays child view controllers that
control the top of the screen, it should delegate control of the
status bar to the currently selected child view controller, and ensure
that the status bar is updated when the selected child changes.
material-components/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,willlarche/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,randallli/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,sigito/material-components-ios,material-components/material-components-ios,willlarche/material-components-ios,sigito/material-components-ios,brianjmoore/material-components-ios,material-components/material-components-ios,willlarche/material-components-ios,material-components/material-components-ios,willlarche/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios