Make class loaders weak roots

Making the class loaders weak roots in the class linker prevents them
from keeping the classes as live. However we currently do mark them
as strong roots to make sure no accidental class unloading occurs
until the logic to free from linear alloc is complete.

Bug: 22720414

Change-Id: I57466236d9ce6fd064dda9a30ce8ab68094fb8b0