Open dream customization activity full-screen.

Current behavior is for the 3P dream settings activity to be embedded in
Settings. However, since the 3P activity may have a different theme than
Settings, we should open it in a separate fullscreen activity.

Test: locally on device
Bug: 215733289
Change-Id: I9ced996b47cede874a5c6e8ac0c84914693f527c