CameraRoll.saveImageWithTag()

Saves the image to the camera roll / gallery.

On Android, the tag is a local URI, such as "file:///sdcard/img.png".

On iOS, the tag can be one of the following:

• local URI
• assets-library tag
• a tag not matching any of the above, which means the image data will be stored in memory (and consume memory as long as the process is alive)

Returns a Promise which when resolved will be passed the new URI.