Add function to AppWidgetService for updating widget visibility

Currently, when the widget host stops listening to the widget updates,
the callback will be set to null and the updates to the widget will be
stashed on the service side. Additionally, the visibility of the widget
will be changed. For an upcoming change to the launcher side, a function
that only changes the visibility should be added.

Test: N/A
Bug: 235358918
Change-Id: I9a0f244ca76b378d9c337e0a364bca5955f5ae96