lowmem: Move oom level to libsyscommon

lowmem: Move oom level to libsyscommon

enum oom_level needs to be referenced by both resourced lowmem module and
resourced plugin-backend (i.e., LMK governor). Therefore, it should be located
a shared library (i.e., libsyscommon) which can be accessed by both sides.
In addition, add corresponding header file from libsyscommon.

Change-Id: I1f6c9676e1cdf9f436cbaa5413ed433dfe6e4ffe
Signed-off-by: Unsung Lee <unsung.lee@samsung.com>