Fix google-explicit-constructor warnings in art.

* Add explicit keyword to conversion constructors,
  or NOLINT for implicit converters.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I1e1ee2661812944904fedadeff97b620506db47d
diff --git a/runtime/experimental_flags.h b/runtime/experimental_flags.h
index 7faa2dc..54d2c35 100644
--- a/runtime/experimental_flags.h
+++ b/runtime/experimental_flags.h
@@ -31,7 +31,8 @@
   };
 
   constexpr ExperimentalFlags() : value_(0x0000) {}
-  constexpr ExperimentalFlags(decltype(kNone) t) : value_(static_cast<uint32_t>(t)) {}
+  constexpr ExperimentalFlags(decltype(kNone) t)  // NOLINT, implicit
+      : value_(static_cast<uint32_t>(t)) {}
 
   constexpr operator decltype(kNone)() const {
     return static_cast<decltype(kNone)>(value_);