Store concrete manager
Store concrete manager

manager_t is movable, concrete RAII object;  Inside object_manager,
there is no reason it needs to be stored by unique_ptr.  Move to storing
the manager by value instead of by pointer.

Signed-off-by: Ed Tanous <10f85ce4e203a350849b77be4d3c7d00f8cca579@google.com>
Change-Id: I97206fd3a4cfea43ab6836f45b92a1bb2adb5c32

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus