Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IDrm.cpp b/media/libmedia/IDrm.cpp
index b1ad0c5..284e99a 100644
--- a/media/libmedia/IDrm.cpp
+++ b/media/libmedia/IDrm.cpp
@@ -60,7 +60,7 @@
 };
 
 struct BpDrm : public BpInterface<IDrm> {
-    BpDrm(const sp<IBinder> &impl)
+    explicit BpDrm(const sp<IBinder> &impl)
         : BpInterface<IDrm>(impl) {
     }