Hi Jenny,
I just want to let you know that while this is possible to implement in Google Web Designer, there was a recent change in the latest iOS version that will make this not work on the latest iOS.
We are working with the Studio team to allow this to work on the DoubleClick platform for inApp ads, ETA July release.
If you want to get a head start and try it out now on an Android device, you can have the animation of the cheese puffs dropping away set up, then add a timeline event to pause the animation (right click in first events layer in the first keyframe the timeline advanced mode and select Add event... and add the pause action, then use the shake event to play the timeline animation (click on stage to deselect any element, click on the Plus button in the Events panel to add an event for page1 > Page Shake > Timeline > Play. I attach the test file for you to check out.
You can also check out a template in Google Web Designer called "App Promo Default Assets App Install Shake to Reveal".
Thanks,
San
Google Web Designer team