Honor ANDROID_ROOT and ANDROID_DATA

Change-Id: I8e43093830a734694bbf7308d08dd18527302270
diff --git a/src/class_linker.cc b/src/class_linker.cc
index 32082d3..9d53f93 100644
--- a/src/class_linker.cc
+++ b/src/class_linker.cc
@@ -568,7 +568,8 @@
 bool ClassLinker::GenerateOatFile(const std::string& dex_filename,
                                   int oat_fd,
                                   const std::string& oat_cache_filename) {
-  std::string dex2oat_string("/system/bin/dex2oat");
+  std::string dex2oat_string(GetAndroidRoot());
+  dex2oat_string += "/bin/dex2oat";
 #ifndef NDEBUG
   dex2oat_string += 'd';
 #endif