Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IMediaPlayerService.cpp b/media/libmedia/IMediaPlayerService.cpp
index 05f8670..6fe916d 100644
--- a/media/libmedia/IMediaPlayerService.cpp
+++ b/media/libmedia/IMediaPlayerService.cpp
@@ -54,7 +54,7 @@
 class BpMediaPlayerService: public BpInterface<IMediaPlayerService>
 {
 public:
-    BpMediaPlayerService(const sp<IBinder>& impl)
+    explicit BpMediaPlayerService(const sp<IBinder>& impl)
         : BpInterface<IMediaPlayerService>(impl)
     {
     }