Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I7e061f68acdde368a3b63c029b928133646ebff2
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index a69b11b..2e17bf7 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -231,7 +231,7 @@
              flinger->setTransactionFlags(eDisplayTransactionNeeded);
          }
      public:
-        DisplayToken(const sp<SurfaceFlinger>& flinger)
+        explicit DisplayToken(const sp<SurfaceFlinger>& flinger)
             : flinger(flinger) {
         }
     };
@@ -2450,7 +2450,7 @@
     class MessageScreenInitialized : public MessageBase {
         SurfaceFlinger* flinger;
     public:
-        MessageScreenInitialized(SurfaceFlinger* flinger) : flinger(flinger) { }
+        explicit MessageScreenInitialized(SurfaceFlinger* flinger) : flinger(flinger) { }
         virtual bool handler() {
             flinger->onInitializeDisplays();
             return true;
@@ -3167,7 +3167,7 @@
     }
 
 public:
-    GraphicProducerWrapper(const sp<IGraphicBufferProducer>& impl)
+    explicit GraphicProducerWrapper(const sp<IGraphicBufferProducer>& impl)
     :   impl(impl),
         looper(new Looper(true)),
         result(NO_ERROR),