Fixed to call computeVisibleRegions() when transparent value changed

When some Surfaces are overlapping and one of them changed to
transparent or opaque, the back Surfaces should appear or
disappear. This patch calls SurfaceFlinger::computeVisibleRegions()
to re-calculate region of each Surface to implement the behavior.

Change-Id: Iffb1caf1b4ce28dff252e114fe5b9b07d9c84a6f