Review FlexLayoutManager measurement for possible performance improvements

FlexLayoutManager is currently looping over the child views to measure the full size of the FlexLayout. It's likely we can work out a way to simply retrieve the root Frame of the FlexLayout and use that measurement instead, which should perform a bit better.

Start by locating FlexLayoutManager and tracing how it loops over child views to measure the full FlexLayout. Compare that path with the root Frame measurement described in the issue, and establish whether the alternative is correct and measurably faster. Done means a validated performance improvement with appropriate evidence, but no test or file is named in the issue.

Written by the indexing model from the issue text.