Tweak performance of background image loading.
ImageLoader core unit tests
Additional tweaks to fadeing of images.

Migrated to use of TransitionalDrawable and setting crossfade to to true
to handle image transitions smoother.  Still some code refactoring to do
but for now it works and unit tests need to be fixed.
Fix failing tests and add additional test for BackgroundBitmapDisplayer
Refactor and clean up BackgroundBitmapDisplayer

Code clean up and variable renaming to make it clearer what is going on.