Add SKIP_SCREENSHOT flag instead of using the window type.

The way WM tell SurfaceFlinger not to screenshot certain layers is by
setting the window type WINDOW_TYPE_DONT_SCREENSHOT. This is a bit
confusing since window type represents the type of window. Instead, use
a new flag called SKIP_SCREENSHOT that is explicitly used to tell SF not
to screenshot this layer.

Test: Rounded corners aren't in screenshot
Test: SurfaceViewSyncTest with blast enabled
Fixes: 168943350
Change-Id: Iaafad35e7ce3d15ade25be18dde4862e2d126703