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/instrumentation.cc b/runtime/instrumentation.cc
index 77d29dd..075d225 100644
--- a/runtime/instrumentation.cc
+++ b/runtime/instrumentation.cc
@@ -252,7 +252,7 @@
   }
 
   Instrumentation* instrumentation = reinterpret_cast<Instrumentation*>(arg);
-  UniquePtr<Context> context(Context::Create());
+  std::unique_ptr<Context> context(Context::Create());
   uintptr_t instrumentation_exit_pc = GetQuickInstrumentationExitPc();
   InstallStackVisitor visitor(thread, context.get(), instrumentation_exit_pc);
   visitor.WalkStack(true);