Setup the saving framework to support hiding original photo for editor.

This new approach will try to hide the original image into an auxiliary
directory. User will only see one edited image, while being able to go
back to the original image in the editor.

Refactor SaveCopyTask for more private funcs and more comments.

bug:9468909
Change-Id: I866321d23e6db0b3dbd08fec2a6a7e3142b17b65