Setting BitmapFactory.Options.inMutable to true when available

I ran into a situation where the `Bitmap` supplied to my `Transformation` was immutable.  From Honeycomb we can set inMutable for all decodes to guarantee that the bitmaps will be mutable.  I don't believe this has any negative effects, even if the bitmap is never mutated.

Admittedly, `calculateInSampleSize` is probably not a great place for this, but it served as a nice centralised location for a PoC.  Ultimately this should probably be implemented in a more independent manner, since `calculateInSampleSize` won't always necessarily be called.