Gallery2: Fix detail dialog image count

This commit fixes the displayed image index which needs
to be count up by 1 to make sense for the user.

Before: http://goo.gl/BeRp8Z
After:  http://goo.gl/N3xvX6

Change-Id: Ib8d782188207d9ee7444316e8426b8215c67e594
Merge "Gallery2: Fix detail dialog image count" into jb-mr2