Improve preloading behavior

[ysdong]

Summary: Current behavior won't start preloading items until the first item changes; so if user started scrolling down, preloading won't start till the first item is completely scrolled out of screen

Test plan: build jar; replace the jar we have in the app with the new jar; verify the preloading behavior is updated.

[felipecsl]

does it make sense to submit this PR to the upstream Glide repo instead?

[ysdong]

yep I can do that too. not sure if that takes longer tho

[felipecsl]

yeah we still want the change regardless if they will want to merge into the main library or not