Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IAudioTrack.cpp b/media/libmedia/IAudioTrack.cpp
index 651cb61..6c901ce 100644
--- a/media/libmedia/IAudioTrack.cpp
+++ b/media/libmedia/IAudioTrack.cpp
@@ -47,7 +47,7 @@
 class BpAudioTrack : public BpInterface<IAudioTrack>
 {
 public:
-    BpAudioTrack(const sp<IBinder>& impl)
+    explicit BpAudioTrack(const sp<IBinder>& impl)
         : BpInterface<IAudioTrack>(impl)
     {
     }