Make CodeItem fields private

Make code item fields private and use accessors. Added a hand full of
friend classes to reduce the size of the change.

Changed default to be nullable and removed CreateNullable.
CreateNullable was a bad API since it defaulted to the unsafe, may
add a CreateNonNullable if it's important for performance.

Motivation:
Have a different layout for code items in cdex.

Bug: 63756964
Test: test-art-host-gtest
Test: test/testrunner/testrunner.py --host
Test: art/tools/run-jdwp-tests.sh '--mode=host' '--variant=X32' --debug

Change-Id: I42bc7435e20358682075cb6de52713b595f95bf9
diff --git a/runtime/art_method-inl.h b/runtime/art_method-inl.h
index 869394c..c6c4f4a 100644
--- a/runtime/art_method-inl.h
+++ b/runtime/art_method-inl.h
@@ -459,16 +459,8 @@
   }
 }
 
-inline IterationRange<DexInstructionIterator> ArtMethod::DexInstructions() {
-  CodeItemInstructionAccessor accessor(this);
-  return { accessor.begin(),
-           accessor.end() };
-}
-
-inline IterationRange<DexInstructionIterator> ArtMethod::NullableDexInstructions() {
-  CodeItemInstructionAccessor accessor(CodeItemInstructionAccessor::CreateNullable(this));
-  return { accessor.begin(),
-           accessor.end() };
+inline CodeItemInstructionAccessor ArtMethod::DexInstructions() {
+  return CodeItemInstructionAccessor(this);
 }
 
 }  // namespace art