Ensure PopupDecorView is not IME-focusable during exit transition.

Currently PopupDecorView temporarily gets FLAG_NOT_FOCUSABLE bit during
exit transition.  When doing this we it is important that PopupDecorView
does not have FLAG_ALT_FOCUSABLE_IM bit, because having both
FLAG_NOT_FOCUSABLE and FLAG_ALT_FOCUSABLE_IM means that PopupDecorView
is IME-focusable.

To prevent IMEs from flickering, this CL ensures that PopupDecorView is
not IME-focusable during exit transition.

Bug: 27396330
Change-Id: I0cc81791b826f7d4257f0b6085c028c796fd9536