Make ellipsis START and MIDDLE grapheme cluster aware.

When text contains 2 emojis and each emoji has 2-code units,
the width array will be like [10, 0, 10, 0]. In this case,
without this CL, just the first code unit can be replaced
with the ellipsis marker. This CL makes ellipsis START and
MIDDLE grapheme cluster aware.
Note that ellipsis END is working properly even without
this CL.

Bug: 30613917

Change-Id: I38ff9793f2ea381460c50465c5470e0ad9a405b9