[ALOY-1717] Android: Investigate default project-assets

When working on a new Alloy project, I noticed some weird project structures regarding Android: - We use the DefaultIcon.png to generate the density-specific app-icons, but there still is a app/assets/android/appicon.png which should not be there by default - We generate the density-specific folders, but only from ldpi to hdpi, not xhdpi, xxhdpi and xxxhdpi. Those should be generated as well - Do we even support ldpi resolutions anymore? Which devices with Android 4.4 have that low density values? - Should we have a "default.png" have a 320x480px resolution? How does Android does that natively? I just want to ensure we don't copy something that isn't actually needed.