Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I11fe54e9e968d8a53a66d5b1cefca9026d5a9d7a
diff --git a/media/libmedia/IMediaSource.cpp b/media/libmedia/IMediaSource.cpp
index 84f1181..1db22ec 100644
--- a/media/libmedia/IMediaSource.cpp
+++ b/media/libmedia/IMediaSource.cpp
@@ -106,7 +106,7 @@
 
 class BpMediaSource : public BpInterface<IMediaSource> {
 public:
-    BpMediaSource(const sp<IBinder>& impl)
+    explicit BpMediaSource(const sp<IBinder>& impl)
         : BpInterface<IMediaSource>(impl)
     {
     }