SurfaceFlinger: Prevent screencap command from failure

For secure windows, screen capture fails, and the screencap command
falls back to reading from the framebuffer directly.

Instead, draw a black screen on the framebuffer object if there is a
secure session, so that secure layers cannot be captured.

Change-Id: Ic14ee3f119f622b41f1af3861237204805fb3ca2
CRs-Fixed: 338570