Restore keeping track of desired drawable alpha

- Fix for re-appearance of https://github.com/square/picasso/issues/25 in 2.3.x
- 18c8427 broke alpha support on PicassoDrawable

Tested with SeriesGuide and works fine after these changes. However, I'm not sure if using the super classes `setAlpha()` (lines 113, 115) instead of the local one has any side effects.