Inspired by #504.
Instead of requiring the user to do PLATFORM_ANDROID #ifdefery,
have the android_main entry point exported by raylib and call
the user-defined main. This way many games could (in theory)
run unmodified on Android and elsewhere.

This is untested!

Simplified to match raylib installer package folder structure

Most of this code was developed by students, it requires some additional
review to be used for teaching.

Those games have been developed by students and ported to a common base
template. Some of them still require some review to be consistent with
each other (formatting, variables naming, code structure...)