To work around #301, we banned Bitmaps with configs other than ARGB_8888 from being placed into the BitmapPool and defaulted to always requesting ARGB_8888 Bitmaps.

Unfortunately the framework doesn't always obey inPreferredConfig. GIFs in particular almost always produce Bitmaps with null (hidden) configs.

Since those Bitmaps are banned from the pool, apps displaying a mix of single frame GIFs and other content will end up with a net drain on the BitmapPool, causing jank.

From some limited testing it appears safe to re-use ARGB_8888 Bitmaps to decode single frame GIFs, and to re-use Bitmaps with null configs to decode most image types with inPreferredConfig == ARGB_8888.