Fix google-explicit-constructor warnings in webservd.

* Declare explicit conversion constructors.

Bug: 28341362
Change-Id: I021116a30fc7bc95c5445b1dec0e77a5ca48bc87
Test: build with clang-tidy
diff --git a/libwebserv/binder_response.h b/libwebserv/binder_response.h
index 2d6c738..675290d 100644
--- a/libwebserv/binder_response.h
+++ b/libwebserv/binder_response.h
@@ -28,7 +28,7 @@
 
 class BinderResponse : public Response {
  public:
-  BinderResponse(const android::sp<android::webservd::IHttpRequest>& request);
+  explicit BinderResponse(const android::sp<android::webservd::IHttpRequest>& request);
 
   // Overrides
   void AddHeaders(