Fix activity stack when restarting the screen nail

We were stacking multiple activities, resulting in the need for multiple
destructions (back key events) when killing the camera app

Change-Id: I7ec61c517d25f7e47baf47fcd1696f987631c344
Merge branch 'cm-10.1' of https://github.com/CyanogenMod/android_packages_apps_Gallery2 into jb42