SF: Force refresh when in single buffer mode

- Add a boolean to BufferItem to track whether single buffer mode is
  enabled. When it is, force SurfaceFlinger to acquire a new buffer
  and refresh on every vsync.

Bug 24940410

Change-Id: Iea67330c416b6fb14500865f98c67f1c12f23197
diff --git a/libs/gui/BufferQueueConsumer.cpp b/libs/gui/BufferQueueConsumer.cpp
index 8e2afd0..6f9f21f 100644
--- a/libs/gui/BufferQueueConsumer.cpp
+++ b/libs/gui/BufferQueueConsumer.cpp
@@ -203,11 +203,15 @@
                     (mCore->mSingleBufferCache.transform &
                     NATIVE_WINDOW_TRANSFORM_INVERSE_DISPLAY) != 0;
             outBuffer->mSurfaceDamage = Region::INVALID_REGION;
+            outBuffer->mSingleBufferMode = true;
+            outBuffer->mQueuedBuffer = false;
         } else {
             slot = front->mSlot;
             *outBuffer = *front;
         }
 
+        outBuffer->mSingleBufferMode = mCore->mSingleBufferMode;
+
         ATRACE_BUFFER_INDEX(slot);
 
         BQ_LOGV("acquireBuffer: acquiring { slot=%d/%" PRIu64 " buffer=%p }",