Add bidirectional scroll capture in infinite scroll.

Previously if we never hit a boundary, we would only capture downard.
This captures above and below in this case.

This has to be done by only requesting adjacent blocks, so the algorithm
is:
- Request tiles upward until we hit the top or we get "enough" headroom
for the infinite scroll case.
- If we hit the top, switch directions, acquire images until we hit
 max tiles or the bottom, then finish.
- If we didn't hit the top, Switch directions and see if you can get the
max tiles in the down direction (if so, finish).
- If we hit the bottom before getting max tiles, switch directions again
and go up until we hit max tiles or the top then finish.

Test: Run scroll capture in infinite scroll, run it near the top of a
finite scroll view and near the bottom, observe output. Enjoy.

Bug: 181562681
Change-Id: Ibe2a0f39ea82789003a30924a9e7f55f0fa37a71