Registration: validate phone number format and add country picker

This is not P1, but it will have to be done, to have the same level of functionality than Riot-Android, which uses `com.google.i18n.phonenumbers`.

For the moment, we ask the user to use the international format when the add a phone number during registration, and there is no format validation.

Note that phone number are no longer required or optional during registration on matrix.org homeserver, that's why this issue is not P1.