android: lowmemorykiller: Add whitelist PIDs that never been killed

Add a whitelist parameter that lets user-space specify a set of PIDs (up
to 6) that never been killed. Works in similar way like adj and minfree.

For example:
echo "PID1, PID2" > /sys/module/lowmemorykiller/parameters/w_list

Conflicts:

	drivers/staging/android/lowmemorykiller.c