refactor: Clean up enum preferences

[nikclayton]

After #945 lands there's a framework for managing enums as preferences. Extend that to cover all the existing enums-as-preferences.

This should remove some of the string arrays that are in use for preference values, keeping the values closer to where they're used. It also keeps the code style consistent, so code that does the same thing looks the same.

At least:

• Implement AppTheme as a PreferenceEnum #950
• EmbeddedFontFamily
• UpdateNotificationFrequency

In Account prefs:

• Default post privacy

Others to consider

• Text size (STATUS_TEXT_SIZE)
• Main nav position (MAIN_NAV_POSITION)