Add android prefab script

[madebr]

• Include mpg123 patch to fix building libmpg123 for i686-Android
• libmodplug is not available as a git submodule anymore, so remove the vendoring code from the cmake script
• add script to android prefab archive. Usage of SDL2_image in a gradle android project is demonstrated at https://github.com/madebr/sdlcross. SDL2_mixer needs the same workaround as SDL2_image to make sure external decoders are included in the .apk (see https://github.com/libsdl-org/SDL_image/pull/309) Example android archive: SDL2_mixer-2.7.0.zip

[sezero]

The textrels had gone as of mpg123 v1.25.0: http://mpg123.org/cgi-bin/news.cgi#2017-05-29 Do we know which asm source(s) still have it?

[madebr]

@sezero I've reported it here https://sourceforge.net/p/mpg123/bugs/345

[madebr]

I backported: https://github.com/madebr/mpg123/commit/d76872def7358f9fc17aacf2cdae3b2788a254b6 and https://github.com/madebr/mpg123/commit/9774c569467304e393febb8b58353c2dbe837f88

[sezero]

Is this ready for merge?

[madebr]

It's okay for me. We'll deal with issues if/when we create SDL packages on maven central.