[Android] Document the event callback system used in the Settings menu in large detail.
[Android] Fixed a typo in the event callback system documentation.
[Android] My bad, somehow the Java file in the previous commit got encoded with UTF8+BOM.
[Android] Finish documenting the native functions in NativeLibrary.java.
[Android] Greatly simplify the input handling for the button mapping settings. Now input handling is directly in the fragment.