Fix google-explicit-constructor warnings in utils.

* Declare explicit conversion constructors.
* Use NOLINT(implicit) for implicit conversion constructors.

Bug: 28341362
Change-Id: I4fb58fa32aafc520b2ed862a05a8bdbfabbc6c35
Test: build with WITH_TIDY=1
diff --git a/include/utils/RefBase.h b/include/utils/RefBase.h
index c82e7d9..950dbd0 100644
--- a/include/utils/RefBase.h
+++ b/include/utils/RefBase.h
@@ -222,12 +222,12 @@
     
     inline wp() : m_ptr(0) { }
 
-    wp(T* other);
+    wp(T* other);  // NOLINT(implicit)
     wp(const wp<T>& other);
-    wp(const sp<T>& other);
-    template<typename U> wp(U* other);
-    template<typename U> wp(const sp<U>& other);
-    template<typename U> wp(const wp<U>& other);
+    explicit wp(const sp<T>& other);
+    template<typename U> wp(U* other);  // NOLINT(implicit)
+    template<typename U> wp(const sp<U>& other);  // NOLINT(implicit)
+    template<typename U> wp(const wp<U>& other);  // NOLINT(implicit)
 
     ~wp();