Allow use system monospace font for field typeface

**Is your feature request related to a problem? Please describe.**

Now the field typeface has only two options:
1. Hardcoded Fira Mono font when the "Field Typeface" option is enabled, it's visible but inconsistent.
2. System default font (normally sans-serif variable-width font) when the "Field Typeface" option is disabled, it's consistent but invisible.

**Describe the solution you'd like**

Provide a new option to use system monospace font, in most cases, it can gain visibility and consistency. If the system monospace is not visible enough, users can go to Hardcoded Fira Mono as the last choice.

**Describe alternatives you've considered**

If three options are considered too many, consider replacing the system default font with the system monospace when the "Field Typeface" option is disabled

**Additional context**

Start by locating the "Field Typeface" option and the code that applies typefaces to fields. Confirm how Android's system monospace font can be exposed as an option, then verify that fields use it and that the existing system default and hardcoded Fira Mono choices still behave as intended.

Written by the indexing model from the issue text.