SurfaceFlinger: track the HWC retire fences

This change adds functionality to the HWComposer class to track the retire
fences returned by each hwc set call.  It adds the HWComposer::getDisplayFence
method to allow other parts of SurfaceFlinger to get a fence that will signal
when the most recent flip takes effect on a display.

Change-Id: I676e0f98440fc58ed70908aa1a1e2ee3e7bf9eb2