Use live region for 'saving screenshot' announcement

Using announceForAccessibility is disruptive; switch to setting the
contentDescription on a view to get better behavior for the "saving
screenshot" announcement.

Bug: 383642111
Flag: com.android.systemui.screenshot_announce_live_region
Test: manual (with talkback on)
Change-Id: I4d3e5bb2bfe6021d280a2b2f3cbf2cc590cfddf7