audiod: Make stat_task static.

The only reason it wasn't static was that audiod_command.c
could use it. However, audiod_command.c only needs
stat_item_values[] and several other fields that were only
used in get_time_string().

So move stat_item_values[] out of struct status task so that
audiod_command.c can use it, and move get_time_string() from
audio_command.c to audiod.c.