Previously, the image overlay would always be loaded asynchronously.
Given clicking left/right opens a new mediaviewer this left a jarring
white flash. By loading the image overlay synchronously after the first
has been loaded, we avoid this flash.
Note, the task does propose preloading and animating the images
but this is a much bigger change and deemed out of scope for this
particular task.
Additional change:
* Use a shared mw.Api instance for ImageOverlay and PageGateway
Bug: T197110
Change-Id: I28d06b34cdea4fedcd7fb754572191e904ecc81a
ba064c2013