Bug 900848 - Use withheld-number string key instead of unknown in call log

Also remove a portion of dead code that was referring to the unknown
key: since |Utils.getPhoneNumberPrimaryInfo| will return null in case
|matchingTel| is invalid, it means that either way we will have been
thrown out of this code previously, since the |matchingTel| is being
checked for in the if() code just above.
Merge pull request #11421 from lissyx/bug900848

Bug 900848 - Use withheld-number string key instead of unknown in call log r=etienne