lowmem-monitor: Split up lowmem-monitor feature from lowmem module

lowmem-monitor: Split up lowmem-monitor feature from lowmem module

The lowmem-monitor.c was created by extracting the monitor feature from
lowmem.c. (The handler of lowmem pressure)
    * Newly created: lowmem-monitor.c, lowmem-monitor.h
    * "lowmem_monitor_pressure_initialize" function of
      lowmem-monitor.c initializes the handler for lowmem pressure.
    * Symbols used only in lowmem-monitor.c were moved from lowmem.c to
      lowmem-monitor.c.
    * The functions used in both lowmem-monitor.c and lowmem.c remain in
      lowmem.c and were changed to 'non-static' functions.

Change-Id: I26bad0c629b74e8261393d4f7a1d5ce60a9215ab
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>