DexFile: Add basic support for v38 dex files.

Makes sure they aren't rejected, essentially. Also adds positive
and negative test cases in dex_file_test.

bug: 30550796
Change-Id: I8c95055a22f8a1435868c8bf3497f9641b7bb900
test: make test-art-host
diff --git a/runtime/dex_file.h b/runtime/dex_file.h
index 3dffe4b..2eca495 100644
--- a/runtime/dex_file.h
+++ b/runtime/dex_file.h
@@ -63,7 +63,7 @@
   static const uint32_t kClassDefinitionOrderEnforcedVersion = 37;
 
   static const uint8_t kDexMagic[];
-  static constexpr size_t kNumDexVersions = 2;
+  static constexpr size_t kNumDexVersions = 3;
   static constexpr size_t kDexVersionLen = 4;
   static const uint8_t kDexMagicVersions[kNumDexVersions][kDexVersionLen];