Fix scroll capture in infinite scroll apps

Previously it would be scrolling down looking for a boundary but not
stop if it hit the max tile count, which would break the scroll capture
system. In reality, if it *ever* hits max tile count, there's nothing
more to be done.

Also added more debug logging to help diagnose any issues.

Unit testing to come in after ag/13766690 lands.

Bug: 182044708
Test: Verify output at the bottom, top and middle of a finite scroll
(Settings), verify output in infinite scroll (Reddit).

Change-Id: I0e70c554f9d4b3ebab4e9cf1003751a44aa5a81e