Mark scenes as ready in a LaunchedEffect

This CL marks seen ready once they have been fully composed, laidout and
drawn once, in a LaunchedEffect. Previously, we were doing that after
drawing all scenes, but this assumed a drawing order which is not
necessarily correct when using layers (e.g. when using scrollable
lists).

Bug: 291053742
Test: Manual, there is no flicker when going from lockscreen to shade
anymore.

Change-Id: Ic6a336196a8892eeb713ebe71cef013b6f88ecb3