LayoutAnimations: fix index consistency crash

Summary:
Unfortunately LayoutAnimations index adjustment is a bit tricky. There was one (hopefully only one!) remaining issue where a specific series of delayed removes, inserts, and removes would trip up index adjustment. Namely in this case: Preexisting delayed removes, and then a later animated commit with immediate inserts followed by higher-index removes.

This case is now resolved and I took time to test and verify other index adjustment paths.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D22709915

fbshipit-source-id: 16ba5bb358608d384f44628dbb6cc691deb8164a