Remove STL from the blueprint level

Removes the STL defined in the LockscreenContent and use the scene scope
that is provided on the top level. In KeyguardViewConfigurator, we need
to make an STL at the root level so that we can invoke the composable
with a SceneScope.

Bug: 301968149
Test: Do transitions in flexiglass
Flag: ACONFIG com.android.systemui.compose_lockscreen DEVELOPMENT
Change-Id: I476b34e7d10f5a4ecab80967f3c9d78d8551f8bc