Set reserved size in Gralloc4 to 0

We are not using the reserved size at the moment.  Set to zero

Bug: 150908307
Test: build, boot, GraphicBuffer_test, GraphicBufferAllocator_test
Change-Id: I673063671afcae91bd659279a34169848b89763a
diff --git a/libs/ui/Gralloc4.cpp b/libs/ui/Gralloc4.cpp
index 6fd4b80..d8e4059 100644
--- a/libs/ui/Gralloc4.cpp
+++ b/libs/ui/Gralloc4.cpp
@@ -77,6 +77,7 @@
     outDescriptorInfo->layerCount = layerCount;
     outDescriptorInfo->format = static_cast<hardware::graphics::common::V1_2::PixelFormat>(format);
     outDescriptorInfo->usage = usage;
+    outDescriptorInfo->reservedSize = 0;
 }
 
 } // anonymous namespace