ICU-11467 Define the 'r' pattern char and the RELATED_YEAR field.

ICU-11467 Define the 'r' pattern char and the RELATED_YEAR field.

For compatibility with ICU4C, the RELATED_YEAR field should be defined
as the field preceding the TIME_SEPARATOR field but by mistake support
for the TIME_SEPARATOR field has already been implemented in ICU4J
without first defining the RELATED_YEAR field.

This change therefore reorders the existing TIME_SEPARATOR field and
adds definitions (without any underlying implementation) for the 'r'
pattern char and the RELATED_YEAR field.

R=markus.icu@gmail.com,pedberg@apple.com

Review URL: https://codereview.appspot.com/198460043

X-SVN-Rev: 37032