resource: process-group: Add aggregated attributes for accounting stats

resource: process-group: Add aggregated attributes for accounting stats

To provide aggregated accounting information about the process group
which includes target process and its child at a driver level, the
structure managing the current process group is changed from list to
hash, and it is preserved until root target is changed or a prcess in
a group is terminated to maintain previous stats. Thus, the clients can
retrieved aggregated accounting stats without create the individual
process driver. Newly added attributes are as follows:
 - PROCESS_GROUP_ATTR_CPU_UTIL
 - PROCESS_GROUP_ATTR_DISK_READ_BPS
 - PROCESS_GROUP_ATTR_DISK_WRITE_BPS
 - PROCESS_GROUP_ATTR_MEM_VIRT
 - PROCESS_GROUP_ATTR_MEM_RSS

Change-Id: I967da6110fb8568383c2ba096d599f51ce8e54e2
Signed-off-by: Dongwoo Lee <dwoo08.lee@samsung.com>