Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I625fbab25f16282c2ea929cd16283d9d146fa17b
diff --git a/media/libmedia/IStreamSource.cpp b/media/libmedia/IStreamSource.cpp
index 840e453..8ec521c 100644
--- a/media/libmedia/IStreamSource.cpp
+++ b/media/libmedia/IStreamSource.cpp
@@ -51,7 +51,7 @@
 };
 
 struct BpStreamSource : public BpInterface<IStreamSource> {
-    BpStreamSource(const sp<IBinder> &impl)
+    explicit BpStreamSource(const sp<IBinder> &impl)
         : BpInterface<IStreamSource>(impl) {
     }
 
@@ -141,7 +141,7 @@
 ////////////////////////////////////////////////////////////////////////////////
 
 struct BpStreamListener : public BpInterface<IStreamListener> {
-    BpStreamListener(const sp<IBinder> &impl)
+    explicit BpStreamListener(const sp<IBinder> &impl)
         : BpInterface<IStreamListener>(impl) {
     }