mgmt/mcumgr/lib: Optimize taskstat for Zephyr

The commit changes taskstat code to directly process thread information
from Zephyr structures, instead of translating them to system agnostic
layer, before formatting response.
It also moves the takstat code to os_mgmt.c.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>