[core] Revert workaround for std::atomic on ARMv5

Got fix on r12.

https://github.com/android-ndk/ndk/issues/31

platform/android/src/async_task.cpp

@@ -1,9 +1,9 @@
 #include "run_loop_impl.hpp"
 
 #include <mbgl/util/async_task.hpp>
-#include <mbgl/util/atomic.hpp>
 #include <mbgl/util/run_loop.hpp>
 
+#include <atomic>
 #include <functional>
 
 namespace mbgl {
@@ -45,7 +45,7 @@ private:
 
     // TODO: Use std::atomic_flag if we ever drop
     // support for ARMv5
-    util::Atomic<bool> queued;
+    std::atomic<bool> queued;
     std::function<void()> task;
 };