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/compiler/dex/dex_to_dex_decompiler_test.cc b/compiler/dex/dex_to_dex_decompiler_test.cc
index 89a63c0..8bccd3b 100644
--- a/compiler/dex/dex_to_dex_decompiler_test.cc
+++ b/compiler/dex/dex_to_dex_decompiler_test.cc
@@ -99,7 +99,7 @@
         if (compiled_method != nullptr) {
           table = compiled_method->GetVmapTable();
         }
-        optimizer::ArtDecompileDEX(updated_dex_file,
+        optimizer::ArtDecompileDEX(*updated_dex_file,
                                    *it.GetMethodCodeItem(),
                                    table,
                                    /* decompile_return_instruction */ true);