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
Gallery2: Clear the undefined buffer for the first 15 frames

The view is undefined before the first frame of camera preview data
come up.
This makes the UI has random corruption when open camera
and exit quickly.
However, we have no way to know the timing of the first frame.

It will take about 700~800ms before first camera preview data comes up,
and the time of drawing one frame is about 50ms,
so need to clear 15 frames before first camera preview data comes up.

Change-Id: Ibc9a5662a15ad58919384ce00cb3a7557b17cae8
Camera: Add checker for null pointer exception during camera release

In current code, handler will still try to execute command even
camera is closed, so add null point checker to skip these commands

Change-Id: Idc407b31bedbf5e879673cb2ca4011dbb006587a