Use command line android root in dex2oat to find image.

Change-Id: I20a025e07da3792131d7126ef0e5863b923c794b
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index 35149cf..5a42147 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -1024,7 +1024,7 @@
 
   bool image = (!image_filename.empty());
   if (!image && boot_image_filename.empty()) {
-    boot_image_filename += GetAndroidRoot();
+    boot_image_filename += android_root;
     boot_image_filename += "/framework/boot.art";
   }
   std::string boot_image_option;