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/signal_catcher.cc b/runtime/signal_catcher.cc
index 611c0a8..c13776d 100644
--- a/runtime/signal_catcher.cc
+++ b/runtime/signal_catcher.cc
@@ -106,7 +106,7 @@
     PLOG(ERROR) << "Unable to open stack trace file '" << stack_trace_file_ << "'";
     return;
   }
-  UniquePtr<File> file(new File(fd, stack_trace_file_));
+  std::unique_ptr<File> file(new File(fd, stack_trace_file_));
   if (!file->WriteFully(s.data(), s.size())) {
     PLOG(ERROR) << "Failed to write stack traces to '" << stack_trace_file_ << "'";
   } else {