Delete gender-balanced emoji sequence by one backspace key event.

The gender-balanced emojis are made with ZWJ sequence and emoji
modifiers.  For example, U+1F469 U+1F3FD U+200D U+1F4BC should be
deleted at the same time by single backsapce key event.  Here, U+1F469
is WOMAN, U+1F3FD is EMOJI MODIFIER FITZPATRICK TYPE-4, U+200D is ZERO
WIDTH JOINER, U+1F4BC is BRIEFCASE.

This CL also renames the state name from STATE_BEFORE_ZWJ_EMOJI to
STATE_BEFORE_EMOJI since now all emoji can be a part of ZWJ sequence
after I572dad42ee108476962d4b3fe9f3a6019cb50098

BUG: 29728397
Change-Id: Ib114295db45c6592f1c65a0773ab236f8bf35209