Set divider visibility to false when #prepareExitSplitScreen.

When we are using #prepareExitSplitScreen, it is because we need the wct to be supplied with exit-split changes, yet StageCoordinator will not be the one driving the transition - someone else is (e.g. desktop). If that's the case, we don't have any way for the divider to be set not visible as part of the transition animation; since we are already pretty sure we are exiting Splitscreen (already removing tasks on MainStage and SideStage), just set divider visibility to false early.

Bug: 413131732
Test: Manual. Open splitscreen, use app handle to put one app into desktop - no more divider view visible.
Flag: EXEMPT bugfix
Change-Id: I23565637dc387389c9305a7072eedfc94e55e42f