Fill up a map of bigram addresses for lookup.

We don't want to do a linear search on each terminal when there
may be 100+ bigrams for a given word because that would be
disastrous for performance. Also, we need to resolve each bigram
address anyway.
This change resolves the addresses at first and puts them in a
balanced tree so that lookup will be O(log(n)).

Bug: 6313806
Change-Id: Ibf088035870b9acb41e948f0ab7af4726f2cee24