feat(MatTooltip): Mimic native tooltip behaviour

#### Feature Description

##### If nested elements have `MatTooltip`s applied on them, show the innermost tooltip.

Suppose we have a `button` with a `span` inside it.

```html

Mat Tooltip

Native Tooltip

```

If both of these elements have the `title` attribute applied to them and we hover the `span`, the browser shows `title` text of the span.
However, if we use the `MatTooltip` directive, both tooltips are shown to the user.

[Minimal Reproduction on StackBlitz](https://mat-tooltip-vs-native-tooltip.stackblitz.io)

#### Use Case

Whenever you need to show different tooltips for parent and child elements.

从链接的 StackBlitz 复现开始，检查嵌套元素的 MatTooltip 行为。完成标准是：将鼠标悬停在嵌套的 span 上时只显示其 tooltip，而将鼠标悬停在父 button 本身上时，父 button 的 tooltip 仍会显示。

由索引模型根据 Issue 内容生成。