bug(tooltip): matTooltip leaks memory

This issue has been reported previously, e.g. in #12644

#### Reproduction

Steps to reproduce:
1. Visit https://material.angular.io/components/tooltip/overview
2. Take memory snapshot.
3. Wiggle cursor over anything with an on-hover tooltip. (approx 100 times)
4. Take memory snapshot.
5. Repeat 3 and 4 to see an upward trend in memory usage.


#### Expected Behavior

Memory usage does not increase indefinitely due to toggling tooltip display.

#### Actual Behavior

It does.

#### Environment

- Angular: 10.0.1
- CDK/Material: Material
- Browser(s): Chrome 84.0
- Operating System (e.g. Windows, macOS, Ubuntu): gLinux

从 https://material.angular.io/components/tooltip/overview 中的复现开始，在反复将光标移到带有 hover tooltip 的元素上时比较内存快照。以 #12644 中较早的报告作为背景，调查涉及显示切换的 matTooltip 行为。当反复切换 tooltip 不再导致内存使用量呈无限上升趋势时，即表示完成。

由索引模型根据 Issue 内容生成。