Add _android files for building DartVM on Android

runtime/bin/extensions_android.cc

 // 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.
 
 #include "bin/extensions.h"
 #include <dlfcn.h>
 
 void* Extensions::LoadExtensionLibrary(const char* library_path,
                                        const char* extension_name) {
   const char* strings[] = { library_path, "/lib",
                              extension_name, ".so", NULL };
   char* library_file = Concatenate(strings);
   void* lib_handle = dlopen(library_file, RTLD_LAZY);
   free(library_file);
   return lib_handle;
 }
 
 void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
   void* result = dlsym(lib_handle, symbol);
   if (dlerror() != NULL) return NULL;
   return result;
 }