lowmem-monitor: Modify PSI memory monitoring levels

lowmem-monitor: Modify PSI memory monitoring levels

The levels of PSI memory monitor changed from 5 levels to 3 levels.
 1. PSI_MEM_LEVEL_LOW (low psi, memory stall is small)
 2. PSI_MEM_LEVEL_MEDIUM
 3. PSI_MEM_LEVEL_HIGH (high psi, memory stall is greater than the aboves)
In accordance with the modification above, stall time also changed.

Code modifications are made to accompany the above modification.

Change-Id: I2bc87ae48a6dd7fc2d019e4a8ebece899c81c747
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>