[web] Calling utility functions in Overlay component.

Summary:
The component now takes all the data needed via props and calls utility functions wrapped in useMemo.
Moved type exports to utility file in previous diff.

Test Plan:
Component not rendered yet, but tested visibility.
App builds.

Reviewers: tomek, atul, kamil

Reviewed By: tomek

Subscribers: ashoat, kamil, atul, tomek

Differential Revision: https://phab.comm.dev/D5720