Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IMediaPlayerClient.cpp b/media/libmedia/IMediaPlayerClient.cpp
index d608386..bbf8475 100644
--- a/media/libmedia/IMediaPlayerClient.cpp
+++ b/media/libmedia/IMediaPlayerClient.cpp
@@ -30,7 +30,7 @@
 class BpMediaPlayerClient: public BpInterface<IMediaPlayerClient>
 {
 public:
-    BpMediaPlayerClient(const sp<IBinder>& impl)
+    explicit BpMediaPlayerClient(const sp<IBinder>& impl)
         : BpInterface<IMediaPlayerClient>(impl)
     {
     }