memory-cgroup: Move code to correct module and refer to correct module

memory-cgroup: Move code to correct module and refer to correct module

Move lowmem module code which is located in memory-cgroup to lowmem module.
In addition, add a function which sets lowest oom score of memory cgroup.
Previously, swap module was incorrectly referencing oom level
insteaf of memory cgroup.

Relocation is done like below:
  - oom_level is only related to lowmem module. so move all related functions
    to lowmem module
  - swap module needs to reference memory cgroup to move pid
    to Throttling memory cgroup. Therefore, it should reference function
    from memory cgroup instead of lowmem module.

Change-Id: Idd863113f7d058afa60973aea541d5c1c03a2c02
Signed-off-by: Unsung Lee <unsung.lee@samsung.com>