dex2oat: Accept .dm files.

And locate primary.vdex in it to do fast verification.

bug: 63920015
Test: 674-HelloWorld-Dm
Change-Id: If920a6c7e4856a047a24bd30b049ef1ee16d7c1f
diff --git a/runtime/vdex_file.h b/runtime/vdex_file.h
index 4e45128..36fe09c 100644
--- a/runtime/vdex_file.h
+++ b/runtime/vdex_file.h
@@ -97,6 +97,9 @@
     friend class VdexFile;
   };
 
+  // Note: The file is called "primary" to match the naming with profiles.
+  static const constexpr char* kVdexNameInDmFile = "primary.vdex";
+
   typedef uint32_t VdexChecksum;
   using QuickeningTableOffsetType = uint32_t;