Fixed Issue#423 - Can't add image from camera

This issue was unrelated to the type of device or the Android version running on it.
The camera images could only be added if the tmp/ folder inside the root folder existed.
Since the tmp/ folder could potentially contain a users project, I fixed this issue by saving the temporary image in the root folder.

Jenkins runs:

https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/184/#showFailuresLink
https://jenkins.catrob.at/job/Catroid-single-UI-device/25/console