fix incorrect nested review counts in v2 scheduler

[dae]

This backports f50cd43 from 2.1.41, allowing AnkiDroid to use the backend deck tree code with the V2 scheduler (will follow up with a patch to AnkiDroid)

[mikehardy]

looks great

[mikehardy]

Okay, I have no rights in this repo, and running them outside of the ankidroid org is no way to go so:

1- I forked ankitects/anki into ankidroid org
2- I added typical ankidroid org teams to access control for ankidroid/anki (so now we have normal non-single-point-of-failure collaboration up and running)
3- I pulled ankidroid and ankidroid-2-1-34 branches from david-allison/anki repo and pushed them to ankidroid/anki (so now we have all ankidroid-specific changes current as of today in new fork)
4- I posted ankidroid#1 to supercede this, so your work is captured in identical PR, but in a place I can merge it
5- I posted a PR to use new anki fork/branch ref as submodule in Ank-Android-Backend ankidroid/Anki-Android-Backend#199

Next steps will be to merge item 5 above, then merge item 4

Will have to figure out how to publish later

This may be closed.