Bouncer: Remove flicker of KeyguardMessageArea

SetBouncerVisible to be bouncerIsShown and not bouncerIsOrWillBeShowing.

This prevents flickering on multiple cases when bouncer is about to
show.

Also rename SetBouncerVisible -> SetBouncerShown for consistency.
Bug: 225182858, 225182858
Test: Manual on device

Change-Id: Ia8e2c114b6cb9565304dd380430622a94400cd8a