Fix two nasty bugs with surrogate pairs.

The important bug is in findWordInTree. The problem, which is
not obvious, is that we were calling codePointAt() with the
code point index in the string, instead of the char index.
The other bug this change fixes was harmless in the practice,
because it's in the iteration which is only used for debug and
pretty printing purposes. It's very similar in that it would
substract a length in code point to a length in chars and
truncate a StringBuilder at that length, so it would fail in a
quite similar manner. This changes the meaning of the "length"
attribute in Position, but it's clearer this way anyway.

Bug: 8450145
Change-Id: If396f883a9e6449de39351553ba83f5be5bd30f0