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
Remove useless/repetitive code

fixScale(float) method has been extracted from the code but the original
code has been kept in place. Because of this, the animation scale is
"fixed" twice which is not necessary. This patch, simply call
fixScale(float) once and stores the result of it in the scale variable.

Change-Id: I50aeacbc2c13ebbf447506ae4309d50c0e313837