Fix flicker issue in IME.

When IME is being moved as part of a window going away, it could flicker
as it immediately moves behind the window.  Fix this.

Also make the default soft input mode for PopupWindow to be to not change
the IME visibility, since it is a rare pop-up window that should cause your
IME to close.

Change-Id: I0b43e080ad012739e9a3e5842794c778c859ac1a