Fix serialization in CopyOnWriteArrayList.

Previously we were double-serializing the elements field. The behavior was
correct but the serialized data was larger than necessary.

This also addresses martinrb's other comments on the new CopyOnWriteArrayList.

Change-Id: I191e84793301e81b89718330d897bdf990c8daa6
tree: 4b825dc642cb6eb9a060e54bf8d69288fbee4904

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