Dexlayout changes to support offset/size output of items.

- Created offset & size fields for items, initially populated with
  values from original dex file.
- Created index field for indexed items in header.
- Created Item objects for StringData and TypeList.
- Created Collections container to hold unique_ptrs for all objects
- Reorganized EncodedArrayItems to make it easier to collect them
- Added relevant annotations-related items to Collections
- Some size calculations and section offsets left as TODO

Bug: 29921113
Test: mm test-art-host-gtest-dexlayout_test
Change-Id: Id812f104c1930a3ee381e885f353a9a49f29ac9b