Typing in Autosize textarea on Firefox mobile causes unexpected characters to appear

#### Reproduction

Steps to reproduce:
1. Install Firefox mobile (https://play.google.com/store/apps/details?id=org.mozilla.firefox, versionName=68.4.2, versionCode=2015676155)
2. Navigate to https://material.angular.io/components/input/examples (currently version 8.2.3)
3. Scroll down to the the 'Auto-resizing textarea' section
4. Tap in the 'Autosize textarea' box
5. Type a few characters

#### Expected Behavior
Characters appear exactly as typed

#### Actual Behavior
- Extraneous characters appear.
- Looks like the Firefox mobile browser and Angular Material are fighting each other, causing extra characters to appear

#### Environment
- Angular: (whatever https://material.angular.io/components/input/examples is using)
- CDK/Material: 8.2.3
- Browser(s):
- Operating System (e.g. Windows, macOS, Ubuntu):

まず、Auto-resizing textareaを使用して、Angular Material input examples pageのFirefox mobile 68.4.2で問題を再現します。次に、Angular MaterialおよびCDKのソースでautosize textareaの動作を追跡します。入力した文字がそれぞれ1回だけ表示され、再現時に余分な文字が生成されなくなれば完了です。

索引モデルが issue の本文から書いたものです。