Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IMediaMetadataRetriever.cpp b/media/libmedia/IMediaMetadataRetriever.cpp
index dbf524e..3c4dce9 100644
--- a/media/libmedia/IMediaMetadataRetriever.cpp
+++ b/media/libmedia/IMediaMetadataRetriever.cpp
@@ -75,7 +75,7 @@
 class BpMediaMetadataRetriever: public BpInterface<IMediaMetadataRetriever>
 {
 public:
-    BpMediaMetadataRetriever(const sp<IBinder>& impl)
+    explicit BpMediaMetadataRetriever(const sp<IBinder>& impl)
         : BpInterface<IMediaMetadataRetriever>(impl)
     {
     }