Fork GlobalScreenshot into notif vs corner flows

Moves GlobalScreenshot into two different files: GlobalScreenshot
(with the new corner flow) and GlobalScreenshotLegacy (with the
previous notifications flow). This is to allow easier iteration on
the new flow; once it is fully tested, the notification/legacy
version will eventually be removed.

Bug: 137153302
Bug: 147423001
Test: tested old and new flows to ensure no user-visible changes; also
tested smart screenshots in old flow to ensure no behavior change
Change-Id: Ia776e2a7c2e8d0d298ea3e0f18f3419aa2b64415