Fix summary writer resource refcounting memory/open-file leak

A missing unref meant that tf.contrib.summary.create_file_writer() resources would not actually be destroyed after they go out of scope, and the underlying event file would also remain open. This change fixes the bug and adds tests that closing and unref'ing the SummaryWriter will ensure the resource is destroyed and its open file is closed.

PiperOrigin-RevId: 234900531