Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I11fe54e9e968d8a53a66d5b1cefca9026d5a9d7a
diff --git a/media/libmedia/IMediaExtractor.cpp b/media/libmedia/IMediaExtractor.cpp
index f142ccc..2a4c669 100644
--- a/media/libmedia/IMediaExtractor.cpp
+++ b/media/libmedia/IMediaExtractor.cpp
@@ -43,7 +43,7 @@
 
 class BpMediaExtractor : public BpInterface<IMediaExtractor> {
 public:
-    BpMediaExtractor(const sp<IBinder>& impl)
+    explicit BpMediaExtractor(const sp<IBinder>& impl)
         : BpInterface<IMediaExtractor>(impl)
     {
     }