Prior to Ic46c4a357 the Project.NameKey class extended StringKey and
was implicitly serializable because StringKey implements Serializable.
After Ic46c4a357, Project.NameKey is no longer serializable, which
causes an IllegalStateException when attempting to bind a cache that
uses JavaCacheSerializer with Project.NameKey as key or value.
Add back the Serializable interface and add a test to make sure it is
serializable by JavaCacheSerializer.
Bug: Issue 11793
Change-Id: Id465c3a4cd810de477fb86a13185ec452d5d3a40
e0ebf9d3e5