Hold proxy classes live in class linker.

We currently assume that class loaders hold proxy classes live, but
this is not always the case (e.g. class loader gets freeed). This
was resulting in a bug where we were freeing a class when there
was still a live object referencing it.

Bug: 11141694
Change-Id: I318e9fee41c86b7790431d09ba5e83633fab547b