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.cc b/runtime/art_method.cc
index 9005120..7ddaa7e 100644
--- a/runtime/art_method.cc
+++ b/runtime/art_method.cc
@@ -297,9 +297,8 @@
     }
   }
   if (found_dex_pc != dex::kDexNoIndex) {
-    const Instruction* first_catch_instr =
-        Instruction::At(&code_item->insns_[found_dex_pc]);
-    *has_no_move_exception = (first_catch_instr->Opcode() != Instruction::MOVE_EXCEPTION);
+    const Instruction& first_catch_instr = DexInstructions().InstructionAt(found_dex_pc);
+    *has_no_move_exception = (first_catch_instr.Opcode() != Instruction::MOVE_EXCEPTION);
   }
   // Put the exception back.
   if (exception != nullptr) {