Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IDrmClient.cpp b/media/libmedia/IDrmClient.cpp
index 490c6ed..444201f 100644
--- a/media/libmedia/IDrmClient.cpp
+++ b/media/libmedia/IDrmClient.cpp
@@ -35,7 +35,7 @@
 class BpDrmClient: public BpInterface<IDrmClient>
 {
 public:
-    BpDrmClient(const sp<IBinder>& impl)
+    explicit BpDrmClient(const sp<IBinder>& impl)
         : BpInterface<IDrmClient>(impl)
     {
     }