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/rsCppUtils.h b/rsCppUtils.h
index 606046e..3da0d03 100644
--- a/rsCppUtils.h
+++ b/rsCppUtils.h
@@ -65,7 +65,7 @@
     // server has no Vector or String8 classes; implement on top of STL
     class String8: public std::string {
     public:
-    String8(const char *ptr) : std::string(ptr) {
+    explicit String8(const char *ptr) : std::string(ptr) {
 
         }
     String8(const char *ptr, size_t len) : std::string(ptr, len) {