Add overlay.alwaysCompose to STL overlays

This CL adds the same alwaysCompose flag for overlays that was added for
scenes in ag/32025518.

This will be used by Flexiglass given that the LazyContent() API is not
available yet and lazily composing scenes/overlays is too costly in
Flexi.

Bug: 432564828
Test: atest SceneTransitionLayoutTest
Flag: com.android.systemui.scene_container
Change-Id: I84b7374aed6c57bb1fadd80ce161c4d5e5680c2b