lowmem-monitor: Modify monitor as a module

lowmem-monitor: Modify monitor as a module

lowmem-monitor was just a library and initialized by lowmem.c using init
function.
lowmem-monitor is now a module which is registered to module manager of
resourced and initialized as an independent module.

lowmem-monitor calculates the memory state with the current available
memory and calls the lowmem_trigger_memory_state_action function with
memory state as a parameter.

lowmem_check_mem_state is a function that calculates memory state with
available memory. This function is used in oom action so it is located
in lowmem.c.

Change-Id: I9a29e7663a1f8a3ec7bf388f65c2cc97e87f5f48
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>