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

Change-Id: Idd4e2edb26d8f76d017016694fcc0c2a2522a980
Ultramem

Change-Id: If7642f1f4e9e4950f20523b7f54b2c3796971e06
Regular memory

Change-Id: I7ae21e3c696611e2874db66f42f7aad2d2c65763