The preload example model doesn't load on user interaction unless the interaction is first a tap.

### Description
Devices with repro: Samsung S8+, Pixel 4 XL

Go to https://modelviewer.dev/examples/loading/#preload swipe back and forth on the model to interact with it. The model does not load unless a single click / tap is performed first. It's a bit counterintuitive since a tap is not required to interact with models on other examples.

Screen recording:

https://user-images.githubusercontent.com/77503048/104784784-e023db80-573d-11eb-99e2-5d08e9036a5f.mp4

### Version
- model-viewer: v1.4

### Browser Affected

- [x] Chrome, version 87.0.4280.141
- [ ] Edge
- [ ] Firefox
- [ ] IE
- [ ] Safari
### OS

- [x] Android P (9), Android S (12)
- [ ] iOS
- [ ] Linux
- [ ] MacOS
- [ ] Windows