SurfaceFlinger: fix a layer occlusion bug

This change fixes a bug where the window visibility would be computed
before any buffers were available, causing the window to be treated as
non-opaque.  When the first buffer arrived, if both mCurrentOpacity and
the opacity determined by the buffer's format were 'opaque', a
recomputation of the opacity would not be done, and the window would
continue to be treated as non-opaque.  SurfaceFlinger could then
unnecessarily draw fully occluded layers.

Change-Id: I2b95da2f4b50e68d50fc5afd8b772e26e62f58d6
Bug: 5057122