[flexiglass] Inflate QS when SceneContainer is composed

Avoids the problem of QS height not populating fast enough during initial shade expansion after reboot. We now inflate before shade scene ever composes, and use the measuredHeight of qqs, so that the notification scrim is placed correctly.

Bug: 369644622
Test: Verified with logging that QS height is correct during first shade expansion
Flag: com.android.systemui.scene_container
Change-Id: I527619c1d4d96b06cd286369ae64f0faf0ae7bea