Add _android files for building DartVM on Android

runtime/platform/thread_android.h

Index: runtime/platform/thread_android.h
diff --git a/runtime/platform/thread_macos.h b/runtime/platform/thread_android.h
similarity index 88%
copy from runtime/platform/thread_macos.h
copy to runtime/platform/thread_android.h
index bded36e200134a2d6f1b75b5a7aa922c29b0e8d3..e7ca78ed2c5908060b37c8de2e4af2c873b01a5b 100644
--- a/runtime/platform/thread_macos.h
+++ b/runtime/platform/thread_android.h
@@ -2,11 +2,11 @@
 // 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_MACOS_H_
-#define PLATFORM_THREAD_MACOS_H_
+#ifndef PLATFORM_THREAD_ANDROID_H_
+#define PLATFORM_THREAD_ANDROID_H_
 
 #if !defined(PLATFORM_THREAD_H_)
-#error Do not include thread_macos.h directly; use thread.h instead.
+#error Do not include thread_android.h directly; use thread.h instead.
 #endif
 
 #include <pthread.h>
@@ -71,4 +71,4 @@ class MonitorData {
 
 }  // namespace dart
 
-#endif  // PLATFORM_THREAD_MACOS_H_
+#endif  // PLATFORM_THREAD_ANDROID_H_