Always use the legacyActivityStarter until new version is implemented

The new activity starter isn't ready yet, so
let's not use it until it is.

Bug: 308819693
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Test: Enable scenecontainer and observe that the legacy activity
starter methods are being called

Change-Id: I466aa71832402b23d9e3d1a0e0f03637b1352601