[Messages] In participant option menu, overlay display should only dislay unknown number/email once

https://bugzilla.mozilla.org/show_bug.cgi?id=920037

- Re-organize the assignment of OptionMenu params
- Clear params.section if the participant is an unknown participant
- Update tests
    - assert content of params.header value
    - assert that params.section is an empty string for unknown participants

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>