kernel/k_malloc: Decouple k_malloc/k_free from mem_pool

These were implemented in terms of the mem_pool/block API directly
(for complicated reasons, the pointers returned from this API may have
been allocated from allocators other than the single system heap).
Have them use a k_heap instead.

Requires a tweak to one test which had hard-coded an assumption about
the header size.

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