Improve handling phone context in RFC3966 format.

[silvio2402]

Previously, the phone number from the tel-URI was prefixed with the phone context. This meant that for example tel:0791234567;phone-context=+41 would produce +410791234567 which is not a valid phone number (it has national trunk and country code).

These changes try to extract the country code from the phone context and use the corresponding metadata to strip the national prefix from the phone number before prefixing it with the extracted country code.

If the phone context goes beyond just the country code, it still prefixes the phone number part with the phone context.

Context: b/370922064