[Fiber] Reset remaining child lanes after propagating context inside Offscreen (#34658)

Otherwise, when a context is propagated into an Activity (or Suspense)
this will leave work behind on the Offscreen component itself. Which
will cause an extra unnecessary render and commit pass just to figure
out that we're still defering it to idle.

This is because lazy context propagation, when calling to schedule some
work walks back up the tree all the way to the root. This is usually
fine for other nodes since they'll recompute their remaining child lanes
on the way up. However, for the Offscreen component we'll have already
computed it. We need to set it after propagation to ensure it gets
reset.