Fix google-explicit-constructor warnings in binder.

* Add explicit keyword to conversion constructors.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I166ab302b3e842cb6d27de89e85767c459f7d826
diff --git a/include/binder/Binder.h b/include/binder/Binder.h
index f849fd4..3404881 100644
--- a/include/binder/Binder.h
+++ b/include/binder/Binder.h
@@ -80,7 +80,7 @@
 class BpRefBase : public virtual RefBase
 {
 protected:
-                            BpRefBase(const sp<IBinder>& o);
+    explicit                BpRefBase(const sp<IBinder>& o);
     virtual                 ~BpRefBase();
     virtual void            onFirstRef();
     virtual void            onLastStrongRef(const void* id);