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):

Start by reproducing the issue in Firefox mobile 68.4.2 at the Angular Material input examples page, using the Auto-resizing textarea. Then trace the autosize textarea behavior in the Angular Material and CDK sources. Done means typed characters appear exactly once and the reproduction no longer produces extraneous characters.

Written by the indexing model from the issue text.