Initialize art::Atomic's value to 0.

art::Mutex and art::ConditionVariable were DCHECKing that the value was
zero on entering the constructor without ever initializing the value.
Since 0 is a sensible default, make it the default for art::Atomic
rather than initializing for each use.

Bug: 16301104

(cherry picked from commit 6a3f8d93ddf09b5f6667820089e488958cba8361)

Change-Id: Iab69bccac9f904111a72eff8162c7f0ca979a08a
diff --git a/runtime/atomic.h b/runtime/atomic.h
index 5ddafb4..868f32c 100644
--- a/runtime/atomic.h
+++ b/runtime/atomic.h
@@ -256,7 +256,7 @@
 template<typename T>
 class Atomic : public std::atomic<T> {
  public:
-  Atomic<T>() : std::atomic<T>() { }
+  Atomic<T>() : std::atomic<T>(0) { }
 
   explicit Atomic<T>(T value) : std::atomic<T>(value) { }