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/interpreter/interpreter_switch_impl.h b/runtime/interpreter/interpreter_switch_impl.h
index 267df2e..aa0f854 100644
--- a/runtime/interpreter/interpreter_switch_impl.h
+++ b/runtime/interpreter/interpreter_switch_impl.h
@@ -25,6 +25,7 @@
 
 namespace art {
 
+class CodeItemDataAccessor;
 class ShadowFrame;
 class Thread;
 
@@ -32,7 +33,7 @@
 
 template<bool do_access_check, bool transaction_active>
 JValue ExecuteSwitchImpl(Thread* self,
-                         const DexFile::CodeItem* code_item,
+                         const CodeItemDataAccessor& accessor,
                          ShadowFrame& shadow_frame,
                          JValue result_register,
                          bool interpret_one_instruction) REQUIRES_SHARED(Locks::mutator_lock_);