Ads can take longer to render compared to content in the article. When ad is loading, there is no indication to the user and AMP will simply show a blank when loading. We should show a placeholder animation while ad is loading. Apply same animation as #3830.

One issue is that we have no idea how long it takes to load the ad. We use the onload event of the ad iframe, but that often fires within milliseconds.

Possible solution is to have the ad overlay on top the animation (if possible) and hence avoiding the need to dismiss animation when ad begins to actually render.

Looking for other suggestions.

CC @cramforce