[snap]: fix hardcoded app icon

[soumyaDghosh]

Fixes #1514

[soumyaDghosh]

@BLKKKBVSIK @spydon can you look into this. This is relatively small PR.

[BLKKKBVSIK]

LGTM

[Feichtmeier]

I don't know if this was intended, but now the icon changed to a monochrome, half transparent icon from the previous yaru full color app icon

now

before

[soumyaDghosh]

@Feichtmeier that's probably not the monochrome icon, but the new icon. It's taken from here,

https://github.com/ubuntu/app-center/blob/main/packages/app_center/assets/app-center.png

Now, I am not sure, if using the svg version would fix it. And the screenshot I shared was not objected by anyone, so, probably intended

[AsciiWolf]

@soumyaDghosh I am not sure whether using a 90x90 px icon is a good idea. The svg one would probably be better. Or this png one that has better resolution (although it still is a non-standard size).