SurfaceFlinger: screenshots w/ protected buffers

This change modifies SurfaceFlinger's screenshot behavior when a layer
with a protected buffer is visible.  The previous behavior was to simply
fail the screenshot.  The new behavior is to render the screenshot using
a placeholder texture where the protected buffer would have been.

Change-Id: I5e50cb2f3b31b2ea81cfe291c9b4a42e9ee71874