Reduced frequency of blocks for concurrent gc

Immediately return for references that are marked before reference
processing without blocking. Soft references are kept in the queue until
the reference processor stops preserving, after which, all marked
references are removed. Finalizer references will still block on get().

Bug: 15471830
Change-Id: I588fcaef40b79ed7c95a4aa7f4fc2e17ee0c288f