Waiting for wallpaper animation in widget tests only for Nexus Launcher and in postsubmit

This code flakes in postsubmit and presubmit, but the failure is
not reproducible in local and abtd runs. Landing the change to make postsubmit
flake; then I'll use tricks like tracing to debug the cause.

Bug: 313926097
Flag: N/A
Test: presubmit
Change-Id: I77c324d43a2400fe1015f5cd0a4729e34552687c