Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IAudioFlinger.cpp b/media/libmedia/IAudioFlinger.cpp
index 0bf503a..632e6d5 100644
--- a/media/libmedia/IAudioFlinger.cpp
+++ b/media/libmedia/IAudioFlinger.cpp
@@ -89,7 +89,7 @@
 class BpAudioFlinger : public BpInterface<IAudioFlinger>
 {
 public:
-    BpAudioFlinger(const sp<IBinder>& impl)
+    explicit BpAudioFlinger(const sp<IBinder>& impl)
         : BpInterface<IAudioFlinger>(impl)
     {
     }