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