audiod: Avoid delay when closing slot.

Currently we might wait up to a full scheduler interval until a slot
is closed.

This commit changes the pre_select method of the status task to check
whether some slot can be closed. In this case it requests a minimal
delay from the scheduler.

The current try_to_close_slot() is split into two functions,
must_close_slot() and close_unused_slots(). The former function is
called from ->pre_select(). It only checks whether a slot must be
closed but performs no action.

The latter function, close_unused_slots() probes all slots and closes
those for which must_close_slot() returns true.