Remove lock from ClassExt installation procedure.

We were using a lock on the class to ensure that we avoid races in
setting the ext_data_ field of a class object. We replace this with a
CAS of the field in order to prevent deadlocks.

Test: mma test-art-host
Change-Id: Ie436ff9526f2c3b38a9af49c5606a7cee6d718f1