[Partial Screensharing] Add string resources

Adding all the copies that are requied for
the partial screensharing feature.
They were approved by UX/product and PWG.

Screenshot: https://screenshot.googleplex.com/ATYGF69tQuhFX3N.png

Test: presubmit
Bug: 240939253
Change-Id: I4e39eeed78cc784611d85e085052a9da934a8733