samples/userspace/prod_consumer: Use a sys_heap

This code used a sys_mem_pool directly.  Use a new-style heap instead
to do the same thing.

(Note that the usage is a little specious -- it allocates from the
heap but doesn't appear to fill or check any data therein, just that
the heap memory can be copied from the two memory domains.  It's
unclear exactly what this is trying to demonstrate and we might want
to improve the sample to do something less trivial.)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>