Initializes iter to iterate over the account settings.
If key_prefix is not null, only keys whose names start with key_prefix will
be iterated over. After calling this method, one would typically call
get_next to read the settings one by one.
| this |
the AccountService. |
| iter |
an uninitialized AccountSettingIter structure. |
| key_prefix |
enumerate only the settings whose key starts with |