Update the divider visibility when moveToStage

The visibility of the divider should be updated when entered by
moveToStage. This CL updates the visibility of the divider to make
sure the divider is visible.

Bug:279137288
Test: adb shell dumpsys activity service SystemUIService WMShell
      splitscreen moveToSideStage taskId stageId
      atest StageCoordinatorTests
Change-Id: I5a4a841d8578ad9f9a64b397149ca110916d2f38