Update Gallery icons to have a white background. (#5252)

I took the logo_square from:
https://github.com/flutter/identity-assets/tree/master/assets/logo_square
Opened it in preview, gave it a white background
and then dropped it in the icon tool to create
these updated icons.

We may want to use offwhite like we did for Android
but this is sufficient for my demo today.

@hansmuller