libgui: move fence handling into ConsumerBase

This change moves some common fence handling code into the base class for
BufferQueue consumer classes.  It also makes the ConsumerBase class initialize
a buffer slot's fence with the acquire fence every time a buffer is acquired.

Change-Id: I0bd88bc269e919653b659bfb3ebfb04dd61692a0
diff --git a/libs/gui/CpuConsumer.cpp b/libs/gui/CpuConsumer.cpp
index fc4a854..e1a2838 100644
--- a/libs/gui/CpuConsumer.cpp
+++ b/libs/gui/CpuConsumer.cpp
@@ -134,7 +134,7 @@
         CC_LOGE("%s: Unable to unlock graphic buffer %d", __FUNCTION__, slotIndex);
         return err;
     }
-    releaseBufferLocked(slotIndex, EGL_NO_DISPLAY, EGL_NO_SYNC_KHR, Fence::NO_FENCE);
+    releaseBufferLocked(slotIndex, EGL_NO_DISPLAY, EGL_NO_SYNC_KHR);
 
     mCurrentLockedBuffers--;