Calculate max_stashed_blocks correctly for block OTA v3

The updater automatically stashes overlapping source blocks to
make it possible to resume if the command performing the update
is interrupted. However, blockimgdiff.py does not take this into
account and therefore, max_stashed_blocks indicates only the
number of explicitly stashed blocks. Change blockimgdiff.py to
correctly calculate the maximum used stash space, including any
automatically stashed blocks.

Bug: 20309033
Change-Id: Ibf5cb1abc38c54e60a6ad3ce4cbce43556b08ac2