refactor(menu): set animation origin based on overlay position

Uses the menu panel's overlay position to determine the animation origin, instead of the trigger. These changes make it easier to follow and avoid having to correct it explicitly based on the `overlapTrigger` flag, in addition to fixing some issues in nested menus.

Author: crisbeto

src/lib/menu/menu-trigger.ts

@@ -238,13 +238,9 @@ export class MdMenuTrigger implements AfterViewInit, OnDestroy {
    * correct, even if a fallback position is used for the overlay.
    */
   private _subscribeToPositions(position: ConnectedPositionStrategy): void {
-    this._positionSubscription = position.onPositionChange.subscribe((change) => {
-      const posX: MenuPositionX = change.connectionPair.originX === 'start' ? 'after' : 'before';
-      let posY: MenuPositionY = change.connectionPair.originY === 'top' ? 'below' : 'above';
-
-      if (!this.menu.overlapTrigger) {
-        posY = posY === 'below' ? 'above' : 'below';
-      }
+    this._positionSubscription = position.onPositionChange.subscribe(change => {
+      const posX: MenuPositionX = change.connectionPair.overlayX === 'start' ? 'after' : 'before';
+      const posY: MenuPositionY = change.connectionPair.overlayY === 'top' ? 'below' : 'above';
 
       this.menu.setPositionClasses(posX, posY);
     });