Fixes a bug that occurs when runing "build.py" from inside the
"android" directory.

Fixes in-tree build when the script is called as "android/build.py"
and not "./android/build.py".

Due to security vulnerabilities.

Android releases were missing since 0.19.1.

Break the mainloop and terminate run call.

Let the user know that MPD has failed.  Not the best thing to do, but
better than pretending it still runs.

Add class Context which wraps the Android/Java Context class and add a
JNI wrapper for method Context.getCacheDir().

We're not using external storage yet, but as soon as we will, we only
need to read music files, not write them.

Allow library auto-detection without pkg-config.