[Bouncer] Refine entry animation scrimmed.

Bouncer is set to visible before animation begins which causes the
bouncer to flicker.

Bug: 260628037
Fixes: 260404351
Test: Open and close bouncer scrimmed from clicking on settings icon in
notif shade a bunch of times.

Change-Id: I8c1e806e094afa0ad2e5162b28755c6d8ec34efa