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/arena_containers.h b/runtime/base/arena_containers.h
index 68cacd5..2c8aa28 100644
--- a/runtime/base/arena_containers.h
+++ b/runtime/base/arena_containers.h
@@ -132,7 +132,7 @@
         arena_allocator_(arena_allocator) {
   }
   template <typename U>
-  ArenaAllocatorAdapter(const ArenaAllocatorAdapter<U>& other)
+  ArenaAllocatorAdapter(const ArenaAllocatorAdapter<U>& other)  // NOLINT, implicit
       : ArenaAllocatorAdapterKind(other),
         arena_allocator_(other.arena_allocator_) {
   }
@@ -168,7 +168,7 @@
         arena_allocator_(arena_allocator) {
   }
   template <typename U>
-  ArenaAllocatorAdapter(const ArenaAllocatorAdapter<U>& other)
+  ArenaAllocatorAdapter(const ArenaAllocatorAdapter<U>& other)  // NOLINT, implicit
       : ArenaAllocatorAdapterKind(other),
         arena_allocator_(other.arena_allocator_) {
   }