Use SVG (vector) icons directly

[chrisboyle]

Now that minSdk is 21 to get Compose support, this also allows use of vector drawables for icons. Currently all of these button icons have been converted into PNGs in res/drawable-*dpi at 4 different sizes. Using SVGs directly would certainly be more maintainable, and might very slightly reduce the app size.

Note that the icons representing each game, as seen on the chooser, are generated from screenshots (in a similar way to upstream) so cannot benefit from this. The screenshots are of course generated by running the game with these save files and I could imagine generating SVGs instead, but the missing piece of that is an SVG-writing implementation of the Puzzles drawing API, and I'm not convinced it's worth creating one.