Fix google-explicit-constructor warnings in frameworks/rs.

 * Declare explicit conversion constructors.
 * Add NOLINT to implicit conversion constructors.

 Bug: 28341362
 Test: build with clang-tidy

Change-Id: If5114de668c1c8946b8fca9cee4b2b14f95be5f1
diff --git a/cpp/util/StrongPointer.h b/cpp/util/StrongPointer.h
index 0f68615..a9995ba 100644
--- a/cpp/util/StrongPointer.h
+++ b/cpp/util/StrongPointer.h
@@ -65,10 +65,10 @@
 public:
     inline sp() : m_ptr(0) { }
 
-    sp(T* other);
+    sp(T* other);  // NOLINT, implicit
     sp(const sp<T>& other);
-    template<typename U> sp(U* other);
-    template<typename U> sp(const sp<U>& other);
+    template<typename U> sp(U* other);  // NOLINT, implicit
+    template<typename U> sp(const sp<U>& other);  // NOLINT, implicit
 
     ~sp();