Add _android files for building DartVM on Android

runtime/platform/thread.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef PLATFORM_THREAD_H_
 #define PLATFORM_THREAD_H_
 
 #include "platform/globals.h"
 
 // Declare the OS-specific types ahead of defining the generic classes.
-#if defined(TARGET_OS_LINUX)
+#if defined(TARGET_OS_ANDROID)
+#include "platform/thread_android.h"
+#elif defined(TARGET_OS_LINUX)
 #include "platform/thread_linux.h"
 #elif defined(TARGET_OS_MACOS)
 #include "platform/thread_macos.h"
 #elif defined(TARGET_OS_WINDOWS)
 #include "platform/thread_win.h"
 #else
 #error Unknown target os.
 #endif
 
 namespace dart {
@@ skipping 60 matching lines @@
  private:
   MonitorData data_;  // OS-specific data.
 
   DISALLOW_COPY_AND_ASSIGN(Monitor);
 };
 
 }  // namespace dart
 
 
 #endif  // PLATFORM_THREAD_H_