audiod: Move get_play_time_slot_num() to audiod.c.

audiod_status_dump() sends the play time string that corresponds
to the current slot to all connected stat clients. It first calls
the get_play_time_slot_num() helper to obtain the slot number, then
calls get_time_string(slot_num).

Currently the helper function is part of audiod_command.c, hence
we expose the slot_info structure to audiod_command.c. Moving the
helper from audiod_command.c to audiod.c is a first step to make this
structure local to audiod.c.  It also allows to remove the slot_num
parameter from get_time_string().