lowmem: Split up governor from lowmem module

lowmem: Split up governor from lowmem module

The lowmem-governor.c was created by extracting the
"sorting out kill candidate" functionality from lowmem.c.
    * Newly created: lowmem-governor.c, lowmem-governor.h
    * "lowmem_governor_get_kill_candidates" function from
      lowmem-governor.c gets a list of procs/apps and returns a sorted
      list of kill candidates.
    * Functions that only used in lowmem-governor.c were moved into
      lowmem-governor.c from lowmem.c.

Change-Id: Ieba1612c040a9891e2f87f44e6bd1a34251f05ea
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>