Load clipboard image thumbnail in background

Also fixes a bug where copying several things in quick succession could cause the UI to appear to freeze (due to starting and cancelling an exit animation, but not redoing the enter animation).

Bug: 267161222
Fix: 267161222
Test: manual, atest ClipboardOverlayControllerTest

Change-Id: I59de7152cd0655417ee78ee787edb54d30e0e39c