Hide IME after re-parenting IME container to DisplayArea

When IME is reparented to new DisplayAreaGoup, its surface is set visible
again. In order to prevent it fom flashing, it should be explicitly
hidden.
IME will reappear with animation once its attached to new app in new
DisplayArea

Bug: 174533580
Test: Manually using steps in bug
      atest DualDisplayAreaGroupPolicyTest

Change-Id: Icfe3ca6334a400f3241d67b3f5a8f5b08958919c