lowmemorykiller: add a knob to turn on/off the feature

Add an option to turn on/off lowmemorykiller  so that
userspace can enable or disable lowmemorykiller as per requirement.

Change-Id: I539f7e23cdf5540e536569f0a582c9e7fe79c3c4
Signed-off-by: Yogesh Lal <ylal@codeaurora.org>