Miscellaneous calling fixes

[oxtoacart]

For getlantern/engineering#770

I haven't been able to test if this improves the voice quality issues, but it's upgraded to the latest flutter_web_rtc so hopefully that helps with call quality

• All strings are defined using localized message keys like 'my_message_key'.i18n and are defined in en.po.
• All colors are defined using Hex (not using built-in colors), are defined in colors.dart and are not duplicated
• All text styles are defined in text_styles.dart and are not duplicated
• All icons are using the vector resource from Figma (do not use built-in Icons)
• Repeated code has been factored into custom widgets
• Layout looks good in both LTR (English) and RTL (Persian) languages
• If you refactored existing code, have you tested the refactored functionality against the old version to make sure you didn't break anything?
• Do the tests pass? Consistently?
• Did this change improve test coverage?
• Is the code in question being linted? If not, consider adding a linter step to CI. If yes, make sure the linter is happy.
• Have you logged tickets for related technical debt with the label “techdebt”?