Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I11fe54e9e968d8a53a66d5b1cefca9026d5a9d7a
diff --git a/media/libmedia/IMediaDrmService.cpp b/media/libmedia/IMediaDrmService.cpp
index 9b6ecfd..84812dc 100644
--- a/media/libmedia/IMediaDrmService.cpp
+++ b/media/libmedia/IMediaDrmService.cpp
@@ -37,7 +37,7 @@
 class BpMediaDrmService: public BpInterface<IMediaDrmService>
 {
 public:
-    BpMediaDrmService(const sp<IBinder>& impl)
+    explicit BpMediaDrmService(const sp<IBinder>& impl)
         : BpInterface<IMediaDrmService>(impl)
     {
     }