Save scroll position across rotation better

When rotating, we now check for the element that's at the top of the
screen, estimate how far into it we were (by percentage of height),
and return us to the recalculated position after rotating.

Bug: 68683
Change-Id: Ibdd0f59bde682da46040fa3b0a5b8daab4822721
Merge "Save scroll position across rotation better"