Added scrim visibility check.

Scrim view was not shown because the TaskbarScrimViewController did not
check taskbar visibility on initialization. Added the check to fix the
issue.

Fixes: 353583417
Flag: com.android.wm.shell.enable_bubble_bar
Test: manual
Switch task bar to transient mode on overview screen or inside an
application. Expand bubble bar.

Change-Id: Ia8a05802a779a4bac70906e7e52ac6650308c85c