ART: Move dex structs into own header

Separating out the structs from DexFile allows them to be forward-
declared, which reduces the need to include the dex_file header.

Bug: 119869270
Test: m
Change-Id: I32dde5a632884bca7435cd584b4a81883de2e7b4
diff --git a/libdexfile/dex/code_item_accessors_test.cc b/libdexfile/dex/code_item_accessors_test.cc
index 87f4bab..c5891f9 100644
--- a/libdexfile/dex/code_item_accessors_test.cc
+++ b/libdexfile/dex/code_item_accessors_test.cc
@@ -71,7 +71,7 @@
   static constexpr size_t kInsnsSizeInCodeUnits = 5;
 
   auto verify_code_item = [&](const DexFile* dex,
-                              const DexFile::CodeItem* item,
+                              const dex::CodeItem* item,
                               const uint16_t* insns) {
     CodeItemInstructionAccessor insns_accessor(*dex, item);
     EXPECT_TRUE(insns_accessor.HasCodeItem());