Attach immersive confirmation window to the immersive activity root

Before, it is always attached to DisplayContent even if it is a
dual-display-area device. Now, it is attached to the same root Display
Area as the immersive activity.

Bug: 155441180
Test: manually with dual-display-area policy
Change-Id: I674a6ed6af394e6778c1db9d799e403473cac553