Fixed issue with opaque thumbnails being given an alpha channel

Summary:Images cropped using the ImageEditor were always given an alpha channel, even if opaque.

This causes them to be saved as PNGs instead of JPEGs (increasing memory and bridge traffic) and also causes unnecessary blending when they are drawn.

Reviewed By: fkgozali

Differential Revision: D2982682

fb-gh-sync-id: b65f361efd78e35f259836111b38d914bae49847
shipit-source-id: b65f361efd78e35f259836111b38d914bae49847