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/base/allocator.h b/runtime/base/allocator.h
index e48eca9..8d1c982 100644
--- a/runtime/base/allocator.h
+++ b/runtime/base/allocator.h
@@ -116,7 +116,8 @@
 
   // Used internally by STL data structures.
   template <class U>
-  TrackingAllocatorImpl(const TrackingAllocatorImpl<U, kTag>& alloc ATTRIBUTE_UNUSED) noexcept {}
+  TrackingAllocatorImpl( // NOLINT, implicit
+      const TrackingAllocatorImpl<U, kTag>& alloc ATTRIBUTE_UNUSED) noexcept {}
 
   // Used internally by STL data structures.
   TrackingAllocatorImpl() noexcept {