Fix standalone weekday parsing.

Parsing standalone weekdays ('cccc') was treated the same way as
"regular" weekdays ('EEEE') since N.

This ensures that the correct names are used for parsing. This bug
doesn't have any effect in most locales, as most either don't have a
separate standalone version of the weekday or they only differ in
casing.

Also added a test for formatting standalone weekdays. It wasn't broken
but also wasn't tested before.

Tested: libcore/run-libcore-tests libcore.java.text.SimpleDateFormatTest
    org.apache.harmony.tests.java.text.SimpleDateFormatTest
Bug: 30323478

(cherry picked from commit 91116215)
Note that git merged the change wrong and it was manually corrected
(passing "true" to parseWeekday for the standalone case).

Change-Id: I7fff3d36b3f3838d5883341ed5eaf3488906f152