libgui: disallow NULL Fence pointers

This change eliminates the uses of a NULL sp<Fence> indicating that no waiting
is required.  Instead we use a non-NULL but invalid Fence object for which the
wait methods will return immediately.

Bug: 7892871
Change-Id: I5360aebe3090422ef6920d56c99fc4eedc642e48
diff --git a/libs/gui/tests/BufferQueue_test.cpp b/libs/gui/tests/BufferQueue_test.cpp
index 12cbfb0..93f8faf 100644
--- a/libs/gui/tests/BufferQueue_test.cpp
+++ b/libs/gui/tests/BufferQueue_test.cpp
@@ -71,7 +71,7 @@
     sp<Fence> fence;
     sp<GraphicBuffer> buf;
     IGraphicBufferProducer::QueueBufferInput qbi(0, Rect(0, 0, 1, 1),
-            NATIVE_WINDOW_SCALING_MODE_FREEZE, 0, fence);
+            NATIVE_WINDOW_SCALING_MODE_FREEZE, 0, Fence::NO_FENCE);
     BufferQueue::BufferItem item;
 
     for (int i = 0; i < 2; i++) {