Add _android files for building DartVM on Android

runtime/vm/virtual_memory_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 "vm/virtual_memory.h"
 
 #include <sys/mman.h>
-#include <sys/unistd.h>
 #include <unistd.h>
 
 #include "platform/assert.h"
 #include "platform/utils.h"
 
 namespace dart {
 
 // standard MAP_FAILED causes "error: use of old-style cast" as it
 // defines MAP_FAILED as ((void *) -1)
 #undef MAP_FAILED
 #define MAP_FAILED reinterpret_cast<void*>(-1)
 
 uword VirtualMemory::page_size_ = 0;
 
 
 void VirtualMemory::InitOnce() {
   page_size_ = getpagesize();
 }
 
 
 VirtualMemory* VirtualMemory::Reserve(intptr_t size) {
+  // TODO(4408): use ashmem instead of anonymous memory.
   void* address = mmap(NULL, size, PROT_NONE,
                        MAP_PRIVATE | MAP_ANON | MAP_NORESERVE,
                        -1, 0);
   if (address == MAP_FAILED) {
     return NULL;
   }
   MemoryRegion region(address, size);
   return new VirtualMemory(region, NULL);
 }
 
@@ skipping 49 matching lines @@
       prot = PROT_READ | PROT_EXEC;
       break;
     case kReadWriteExecute:
       prot = PROT_READ | PROT_WRITE | PROT_EXEC;
       break;
   }
   return (mprotect(address(), size(), prot) == 0);
 }
 
 }  // namespace dart