mat-menu focuses the second item on Arrow Down when opened with the mouse

#### Bug, feature request, or proposal:
Bug

#### What is the expected behavior?
When opening a menu with the mouse, no items are focused. Pressing arrow down (on the keyboard) in this state (where no item is focused) would focus the first item; pressing arrow up would focus the last item (via wrapping around).

#### What is the current behavior?
Opening a menu with the mouse and then pressing arrow down focuses the second menu item.

#### What are the steps to reproduce?
1. Go to
* https://material.angular.io/components/menu/overview
* or https://v7.material.angular.io/components/menu/overview
* or https://v6.material.angular.io/components/menu/overview
2. Click on the "Menu" button in the basic menu example.
3. Press Down Arrow on the keyboard

Recording: [2019-06-02_19-34-41](https://user-images.githubusercontent.com/5409190/58768724-7d33ff80-856d-11e9-9496-452b95939d56.gif)

#### What is the use-case or motivation for changing an existing behavior?
The current behavior is non-intuitive: Nothing is visibility focused, but the menu behaves as if the first item is focused.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular Material 6.4.7, 7.3.7, and 8.0.0
Tested in Firefox and Chrome on Windows 10

#### Is there anything else we should know?
Root cause appears to be that there is always a menu item focused; but, the focused menu item when the menu is opened by the mouse is styled like a normal menu item, giving the impression there is no item focused by default.

This can be observed (at least for 8.0.0) by adding a style for `.cdk-focused.cdk-mouse-focused`, and observing that the style is applied to the first menu item when opening the menu with the mouse.

---

Potentially related issue: https://github.com/angular/components/issues/2657

That issue describes the behavior I see in 5.2.5 (here: https://v5.material.angular.io/components/menu/overview ), where opening the menu with the mouse disables focus styling (making keyboard navigation hard to use), but opening the menu with the keyboard allows for keyboard navigation (whereas now, the focus state is hidden until the user interacts with the keyboard). At the time, the issue was closed saying the behavior was intended.

まず、Angular Material menu overview の基本的なメニュー例で、マウスで開いて Arrow Down または Arrow Up を押したときの動作を再現します。メニューのフォーカス処理を追跡し、マウスで開いた後にどの項目もフォーカスされた状態として扱われていないことを確認します。Arrow Down では最初の項目に、Arrow Up では最後の項目にフォーカスが移るはずです。

索引モデルが issue の本文から書いたものです。