Now we have a proper C++ library, use std::unique_ptr.

Also remove the Android.libcxx.mk and other bits of stlport compatibility
mechanics.

Change-Id: Icdf7188ba3c79cdf5617672c1cfd0a68ae596a61
diff --git a/runtime/thread_pool.h b/runtime/thread_pool.h
index 23bf294..c816c84 100644
--- a/runtime/thread_pool.h
+++ b/runtime/thread_pool.h
@@ -53,7 +53,7 @@
 
   ThreadPool* const thread_pool_;
   const std::string name_;
-  UniquePtr<MemMap> stack_;
+  std::unique_ptr<MemMap> stack_;
   pthread_t pthread_;
 
  private: