Enable SDL2 MP3 support for Android

[AndreMiras]

We had to copy the external/smpeg2-2.0.0/ folder
to the jni/ folder to fix linker errors.
See https://groups.google.com/forum/#!topic/kivy-users/N4isIjQ1Ja8

[AndreMiras]

I wrote "SDK2" in the commit but I meant "SDL2".
It adds MP3 support to Android. It was a lot of investigations because I'm new to the Android toolchain and all, but it doesn't actually do much.
It seems the "external" library must be copied to the jni/ folder if we want it to be linked. This is what it does.

[Fak3]

Why do you split it into 4 methods? It would be more clear if you put it right in prebuild_arch()

[AndreMiras]

Hi @Fak3, thanks for your feedback.
I used "inline methods" because I find it more clear to read, but this is my point of view.
For example when I read the prebuild_arch() method. On first sight to the self._copy_smpeg2_to_jni() line, I can quickly say, OK, the prebuild_arch() just copies the smpeg2 lib to the jni directory. And if I want to see in more details how this was achieved, I can step deeper into the stack.
On the other hand, if I had written:

external_dir = join(self.get_build_dir(None), 'external')
smpeg2_external_dir = join(self._external_dir(), 'smpeg2-2.0.0')
jni_dir = self.get_jni_dir()
shprint(sh.cp, '-r', smpeg2_external_dir, jni_dir)
super(LibSDL2Mixer, self).prebuild_arch(arch)

I fell like it takes more time to understand what the method just does, event though it's a very simple method. It's a little bit like the import antigravity thing.
It's also a way to avoid commenting each single line knowing that comments are future lies.
But again this my point of view and if it's an issue I can do another pull request, merging all these lines into a single method with some comments.
Does the fix work for you by the way?

[Fak3]

I still think that a comment with 2 lines of code will be much easier to read than 3 methods.

# Copy the smpeg2 to the jni dir
smpeg2_dir = join(self.get_build_dir(None), 'external/smpeg2-2.0.0/')
shprint(sh.cp, '-r', smpeg2_dir, self.get_jni_dir())

I have not yet tried it.