Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IMediaPlayer.cpp b/media/libmedia/IMediaPlayer.cpp
index 942aec3..77ca96d 100644
--- a/media/libmedia/IMediaPlayer.cpp
+++ b/media/libmedia/IMediaPlayer.cpp
@@ -72,7 +72,7 @@
 class BpMediaPlayer: public BpInterface<IMediaPlayer>
 {
 public:
-    BpMediaPlayer(const sp<IBinder>& impl)
+    explicit BpMediaPlayer(const sp<IBinder>& impl)
         : BpInterface<IMediaPlayer>(impl)
     {
     }