Store multi byte characters as one element

Getting & setting the cursor is done with byte offsets
instead of data model offset (characters) so we need to
be able to convert between the two as well as splitting
characters.

TODO: The regex only works on surrogate pairs, not
yet combining accents.

fixupInsertion will combine a combining mark with the
character to its left it it can.

Bug: 48630

Change-Id: I8d936fb15d82f73cd45fac142c540a7950850d55
Merge "Store multi byte characters as one element"