After three slow preview requests, switch to showing a bar at the
top of the preview area that contains a manual 'reload' button.
The manual bar will be hidden when an error message is show, but
re-shown again after the error is dismissed. Closing and
re-opening the preview pane doesn't reset the manual mode.
Bug: T304568
Change-Id: Ia72bd1ceab68fdaed5de53137bd8ac5961db4714
