Android builds disabled; replacement character art and more backgrounds

Android builds on my machine have broken, and android builds had previously had some strange problems, so I have disabled them for now. When I have more time, I'll address the issues and re-enable them.

I have cleaned up & colored Ryusei's character sketches for all characters other than Shironeko, so most placeholder character art has been replaced. The new art includes some alternate outfits used late in the game.

Cryptovexillogist's new backgrounds are used throughout the Aoi and Koneko routes, and for most of the core loop. (We are currently filling in the remaining core loop backgrounds -- the lunch locations & the Fujinomiya residence -- before moving on to the Kuroneko route and the end game.)

I have not yet finished writing the Kuroneko route, so that's on my plate for the next few weeks.

Most playable portions of the game have no placeholder art remaining, currently. This is a big deal for me.