Handle emojis with emoji-modifier and zwj properly on backspace

Since Unicode 12, emoji modifier sequences (such as human + light skin
tone) can be combined with zwj. However, current backspace state machine
ignores that and stops when one emoji modifier sequence is found. This
CL make the state machine continue exploration as long as a zwj is found
before an emoji modifier sequence.
BYPASS_INCLUSIVE_LANGUAGE_REASON="man" is the name of the emoji

Bug: 171495034
Test: Do the following:
 - Enter an emoji from the "family" subgroup with different skin tones
   combined such as 1F468 1F3FE 200D 2764 FE0F 200D 1F48B 200D 1F468
   "kiss: man, man, medium-dark skin tone, light skin tone"
 - Press the backspace key and confirm that the entire emoji is deleted.
Change-Id: I260f509832cb24486665019d68a25db0ad089d7e