SurfaceFlinger: fix isOpaque check in lockPageFlip

This change fixes an issue where Layer::isOpaque was being called in
lockPageFlip to get the opaqueness for the new buffer before
mActiveBuffer was updated.

Bug: 5321313
Change-Id: Ibb5d7d3f4e9a2b1448e117a484a0d9f9ca5fc9de