Gallery2: Recycle the image when activity is about to close

By calling recycle() on fullImage object, the destructor of the
SkPNGImageIndex class (libskia) will get called which releases all
the memory resources related to that images.

Change-Id: Ic74d9469cabeb07018b76e183f28115c4a93775a
CRs-Fixed: 551617
Merge "Gallery2: Recycle the image when activity is about to close" into jb4.3