Fix NullpointerException in MediaMetadataRetriever

According to the API documentation and the ICS implementation
the setDataSource(String) method throws an
IllegalArgumentException if path is null. In JB a
NullpointerException is thrown instead. This fix restores
the earlier behaviour.

Change-Id: Ic47baadf91076acc227d92d84f6b8d1d6ecd0c03