bug(select): Down-arrow doesn't display in High Contrast mode

[lukeschaefer]

Reproduction

This bug can be replicated in the example documentation itself:
https://material.angular.io/components/select/examples

Steps to reproduce:

1. In Windows, turn on High-Contrast mode
2. Open the examples in Firefox (chrome doesn't work with Windows High Contrast mode)

Expected Behavior

The down-arrow on the right side of the select box should be visible

Actual Behavior

Instead, it looks like a flat line

Environment

• Angular:
• CDK/Material:
• Browser(s): Firefox
• Operating System (e.g. Windows, macOS, Ubuntu): Windows

[crisbeto]

There's a PR for this already, but it wasn't marked as high priority so we never got around to merging it in. #14219