Improve DateFormat.format.

Add support for the 'c' format character, required in many non-English locales.
Reimplement 'c' and 'E', and 'L' and 'M', so they correctly interpret 5-count
pattern characters.

Replace the old incorrect class documentation with a pointer to the
well-maintained libcore equivalent and the Unicode UTS to which these two
implementations are supposed to conform.

Deprecate the useless constants for pattern characters. No one sane is going
to write MONTH + MONTH + MONTH + MONTH instead of "MMMM".

Correct the documentation for getLongDateFormat and getMediumDateFormat.

Also fix DateUtils.getStandaloneMonthString for LENGTH_SHORTEST.

Tests are in https://android-review.googlesource.com/53291.

Change-Id: I4dda8b18070f05ccdc11c1f0a9381a9d233db4e8