Refactoring and groundwork to fix a bug with older apps

This has a good, although small, impact on performance : it removes
a two-way IPC call in a most frequent case, while possibly adding
one in a rather unfrequent and less critical case.

Also, this fixes a bug with surrogate pairs. This specific branch
of code now correctly handles surrogate pairs.
Aside from this, it should have no impact on behavior.

However, since it does delay access to the previous character in
the text view by a two-way IPC call, it actually goes a long way
toward fixing bug#6668226. It is not really a fix and the race
condition still exists, but this change makes it much, much
harder to hit.

Bug: 6668226
Change-Id: Id11cc6a0b7488d6bd392227cafdcf3a8d4c62f6c