Update screenshot shelf UI flag name

Flipping the old flag turned it on in old builds (where the
experience is not ready yet); changing the flag name and re-rolling it
out.

Bug: 329659738
Flag: NONE
(changes now flagged under com.android.systemui.screenshot_shelf_ui2)
Test: manual (device with the old flag has the old UI; flipping the new
flag gets the new (and not broken) UI)

Change-Id: I8250035bf8d33b977088a7aa748d780de306a7e0
Merged-In: I8250035bf8d33b977088a7aa748d780de306a7e0