Remove oat file location in the image.

The oat file is now always in the same directory, and has the
same name as the image file. Only difference is the extension.

This also removes the need for host-prefix.

Change-Id: I16d1f7aeb1d58372d41921694664e9c321afc1ad
diff --git a/compiler/image_test.cc b/compiler/image_test.cc
index 619b056..05d6693 100644
--- a/compiler/image_test.cc
+++ b/compiler/image_test.cc
@@ -42,7 +42,9 @@
 };
 
 TEST_F(ImageTest, WriteRead) {
-  ScratchFile tmp_elf;
+  // Create a root tmp file, to be the base of the .art and .oat temporary files.
+  ScratchFile tmp;
+  ScratchFile tmp_elf(tmp, "oat");
   {
     {
       jobject class_loader = NULL;
@@ -75,7 +77,7 @@
   UniquePtr<File> tmp_oat(OS::OpenFileReadWrite(tmp_elf.GetFilename().c_str()));
   ASSERT_TRUE(tmp_oat.get() != NULL);
 
-  ScratchFile tmp_image;
+  ScratchFile tmp_image(tmp, "art");
   const uintptr_t requested_image_base = ART_BASE_ADDRESS;
   {
     ImageWriter writer(*compiler_driver_.get());