Fix IME container surface parent for dual-display-area

1. Loose the requirement of FEATURE_IME_PLACEHOLDER that the device can
   choose to not reparent the IME container based on their UX design.
2. Make sure the IME surface parent is up-to-date if the IME container
   window is reparented.

Bug: 243842191
Test: atest WmTests:DualDisplayAreaGroupPolicyTest
Change-Id: Ie51548a007a8eabd274a35db7dbfc26e3c956216