Remove DexFile direct accesses to CodeItem

Motivation: StandardDexFile and CompactDexFile should be able to a
have different layout for code items.

Also addressed comments from a previous CL.

Bug: 63756964
Test: test-art-host

Change-Id: I5ea7a853b8095f68b4443ded0f599f2ac5efbd3a
diff --git a/runtime/dex_to_dex_decompiler.h b/runtime/dex_to_dex_decompiler.h
index 804b7e8..a2e3f28 100644
--- a/runtime/dex_to_dex_decompiler.h
+++ b/runtime/dex_to_dex_decompiler.h
@@ -29,7 +29,7 @@
 // to non-const has too many repercussions on the code base. We make it
 // consistent with DexToDexCompiler, but we should really change it to
 // DexFile::CodeItem*.
-bool ArtDecompileDEX(const DexFile* dex_file,
+bool ArtDecompileDEX(const DexFile& dex_file,
                      const DexFile::CodeItem& code_item,
                      const ArrayRef<const uint8_t>& quickened_data,
                      bool decompile_return_instruction);