Fix missing base64 images

Summary:
I discovered failing snapshot tests (T47222928, T47222859). They fail because `<Image>` doesn't work with base64 anymore.

There are two problems that are causing this.

1st is on iOS https://fburl.com/pw246vgw where if the image has 1 frame count, nothing is displayed.

2nd is in https://fburl.com/3im0u38r where if image is not within assets, we don't display it.

Reviewed By: shergin

Differential Revision: D16334151

fbshipit-source-id: 1ea8ef676b7207834ba63f4264e6ef2f05f24b96