audiod: Make struct slot_info local to audiod.c.

audiod: Make struct slot_info local to audiod.c.

Thanks to the previous commit there is only one user of struct
slot_info left in audiod_command.c: decoder_flags(). Moving this
function to audiod.c allows to make the structure private to audiod.c,
along with the slot array and the MAX_STREAM_SLOTS and FOR_EACH_SLOT
macros.

Since audiod_command still needs the decoder flags for the stat command,
the function is made public and renamed to audiod_get_decoder_flags().