On iOS, when a tap is detected in the status bar, provide a means to
pass that touch event through to one or more FlutterViewControllers to
trigger a scroll to top. In iOS apps, scroll to top should occur under
the following conditions:

1. There is one and only one UIScrollView visible with
   scrollsToTop == YES.
2. The status-bar is in standard height mode, not in double-height mode.
   In double-height mode, the expected behaviour is to trigger a switch
   to the application associated with the double-height status bar.
3. A tap or a drag gesture occurs that is entirely constrained to the
   status bar frame. (We currently only handle the tap scenario).

Unfortunately, AppDelegates only get touchesBegan events for status bar
taps, though get get touchesBegan and touchesEnded events for drags
within the status bar frame. As such, we currently synthesise the
touchesEnded event for taps.