[flexiglass] Fixes crashloop in LockscreenScene.

Not sure what had changed, but it looks like LockscreenScene produces an
IAE when attempting to find the views in KeyguardRootView early on. Moving these to happen later fixes the problem.

Bug: 280879610
Test: it no longer crashloops when Compose + Flexiglass is on.
Change-Id: I241ca6bc95f7664a0c4fd6130484a271d0590ef2