SurfaceFlinger: unfreeze windows for fixed size buffers.

This change makes SurfaceFlinger unfreeze a window if it ever gets a
buffer that is fixed-size.  Normally the window would not be frozen if
its in fixed-size mode, but if the window was frozen before entering
fixed-size mode then it should be unfrozen.

Change-Id: I6bc822d4b02ae51fa8914c1f60f5d24b2002b38d