[Dialogs] Fix position of leading/trailing actions in vertical layout

When actions are in vertical layout, the first trailing or leading action is incorrectly positioned on top of the second action, instead of above it.
This is fixed by moving the calculations of the y position of the second button after the first button is positioned.

PiperOrigin-RevId: 318979889

components/Dialogs/src/private/MDCAlertControllerView+Private.m

@@ -1007,15 +1007,13 @@ - (void)layoutVerticalButtons:(NSArray<MDCButton *> *)buttons {
         buttonOrigin.x =
             self.actionsScrollView.contentSize.width - buttonRect.size.width - actionsInsets.right;
       }
-      buttonOrigin.y += multiplier * buttonRect.size.height;
-
       buttonRect.origin = buttonOrigin;
       button.frame = buttonRect;
       if (button != buttons.lastObject) {
         MDCButton *nextButton = buttons[index + 1];
         CGFloat verticalMargin = [self actionsVerticalMarginBetweenTopButton:button
                                                                 bottomButton:nextButton];
-        buttonOrigin.y += multiplier * verticalMargin;
+        buttonOrigin.y += multiplier * (buttonRect.size.height + verticalMargin);
       }
     }
     // Handle RTL