Textures aren't recovered on game resume on Android

[SaurabhTotey]

When running the game on Android, if the game is open and home/back is pressed, and then the game is resumed, a lot of textures are gone. It seems like using an AssetManager in Main should do the trick according to https://github.com/libgdx/libgdx/wiki/Managing-your-assets#resuming-with-a-loading-screen, but I'm not sure, and we'd need to implement this first. Regardless, we should be using an AssetManager as it seems that that is good style in LibGDX and AssetManager handles a lot of asset management correctly.