Update snapshot changes flags when visibility changes

Layerhistory walks through all snapshots and checks the change
flags before updating its state. When a layer moves offscreen,
we update its snapshot but did not update the change flag. This
cl fixes this.

Bug: 345021645
Test: presubmit
Flag: EXEMPT bugfix

Change-Id: Icee06279f0fa8b0f4c0539e99dde8960ba3fab31