Change dex caches to be weak roots

Changed dex caches to be weak roots. This is necessary for class
unloading since the resolved types arrays would keep classes live
when they should be unloaded. Currently the dex caches still don't
get freed due to the class loader roots.

Also deleted some unused functionality in image writer.

Bug: 22720414
Change-Id: If22cb3cad7e3baabc8158a77d7f20799faf4c341