Don't collect lockscreen transition value with Flexi off

We don't need the lockscreen transition value with Flexi off, and it
might be the culprit for some jank regressions, plus the
isUnexpectedlyInLegacyMode call in the flow callback will be less
cranky.

Bug: 332558345
Fixes: 354065727
Fixes: 354081031
Fixes: 354763436
Flag: com.android.systemui.scene_container
Test: atest VisualStabilityCoordinatorTest
Change-Id: I5ae681f7d15a8e4ffbefd9947da8474c8c8439c2