Add constructors for FileDescriptors

[keyboardr]

Adds support for creating Mp3Files with FileDescriptors. Fixes #171

Since FileDescriptors contain less information than Files or Paths, some caveats are needed:

• Saving new files from these Mp3Files is not supported since the absolute path cannot be verified.
• getFilename() and getLastModified() will throw an exception.
• getLength() is only valid after initialization (shouldn't be an issue for those using Mp3File, but it is a concern for FileWrapper).

[hennr]

Thanks for the PR @keyboardr
I am not into android programming, anyone who can offer a review here would be very appreciated.