Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I0ce850de85e0de6a5472dd142f738856142db9ec
diff --git a/libs/gui/IGraphicBufferAlloc.cpp b/libs/gui/IGraphicBufferAlloc.cpp
index 3009989..d4d4702 100644
--- a/libs/gui/IGraphicBufferAlloc.cpp
+++ b/libs/gui/IGraphicBufferAlloc.cpp
@@ -37,7 +37,7 @@
 class BpGraphicBufferAlloc : public BpInterface<IGraphicBufferAlloc>
 {
 public:
-    BpGraphicBufferAlloc(const sp<IBinder>& impl)
+    explicit BpGraphicBufferAlloc(const sp<IBinder>& impl)
         : BpInterface<IGraphicBufferAlloc>(impl)
     {
     }
@@ -90,7 +90,7 @@
     class BufferReference : public BBinder {
         sp<GraphicBuffer> mBuffer;
     public:
-        BufferReference(const sp<GraphicBuffer>& buffer) : mBuffer(buffer) {}
+        explicit BufferReference(const sp<GraphicBuffer>& buffer) : mBuffer(buffer) {}
     };