Fix bug 3058324

PopupWindow now responds to the style attribute popupAnimationStyle
instead of windowAnimationStyle. The latter was being overridden with
undesired values. Animation.PopupWindow style added as a special
default sentinel value, which switches animation styles based on
whether the window is showing above or below an anchor.

Change-Id: Id7cc81f089b6053f2d3a495b1046002548ecf125