Avoid changing scenes if already on the desired scene

This fixes a bug where the KeyguardState override set by calling
changeScene sticks around because the scene never actually changes. This
causes issues the next time a transition occurs, as we end up in the
wrong KeyguardState.

Bug: 361819832
Test: atest CommunalSceneInteractorTest
Flag: com.android.systemui.communal_scene_ktf_refactor
Change-Id: I5f45aaae037cce0e8f844698222a1a58577cbde7