Use live region to announce clipboard status

Switch to using contentDescription on a live region rather than a
disruptive announcement

Bug: 378856296
Flag: com.android.systemui.clipboard_announce_live_region
Test: manual with flag on/off; atest ClipboardOverlayControllerTest
Change-Id: I9af6c8bebbcbd5d0275d968a76b182b3c9c4b34e