bug(cdkTargetMenuAim): directive does not really work

### Description

According to the docs, the `cdkTargetMenuAim` allows for a "Smart Aim" behavior which calculates the intended slope of the mouse pointer and does not switch to another submenu unless some timeout is reached or the mouse deviates from the slope. This behavior does not seem to work at all (tested on several versions: 16.1.2, 17.3.6).

### Reproduction

StackBlitz link: https://stackblitz.com/edit/qwy1he?file=src%2Fexample%2Fcdk-menu-menubar-example.html
Steps to reproduce:
1. Try moving in a diagonal line from the `edit` menu item to the `cut` menu item in the submenu.
2. Doing so will immediately open the `share` submenu instead, no matter how fast the action is done (unless its faster than the refresh rate of the monitor)

### Expected Behavior

That moving the mouse along a diagonal line only opens a different subMenu if the mouse took too long to travel.
see https://rawgit.com/kamens/jQuery-menu-aim/master/example/example.html for an example

### Actual Behavior

The different submenus disappear immediately.

From my limited debugging I was able to glean that the issue seems to stem from the PointerTracker, where the `_getItemPointerEntries()` stream doesn't seem to get output any value on entering children menu items, while the exit is called every time, meaning the `previousElement` and `activeElement` are set as undefined for the duration of the mouse interaction with the menu items and submenus.

### Environment

- Angular: 17.3.6
- CDK: 17.3.6
- Browser(s): Chrome, Firefox
- Operating System (e.g. Windows, macOS, Ubuntu): Windows

リンクされている StackBlitz の再現環境から始め、PointerTracker、特に _getItemPointerEntries() のストリームとその enter/exit 処理を調査します。edit 項目から cut に向かう斜めの移動を再現し、意図したサブメニューが、timeout に達するかポインターが傾斜領域から離れるまで開いたままになることを確認します。

索引モデルが issue の本文から書いたものです。