add overlays to override lowmemorykiller adjustment values

Add overlays config_lowMemoryKillerMinFreeKbytesAbsolute and
config_lowMemoryKillerMinFreeKbytesAdjust to allow a product to
modify the size of the lowmemorykiller minfree buckts.  The absolute
overlay sets the size of the largest bucket to the specified value
and scales the smaller buckets proportionally.  The adjust overlay
(which can be negative) directly adds to the size of the largest
bucket and to the the smaller buckets proportionally.

Change-Id: I0d6b7662be12fd151deb2bf9591f2c7a8b1cb6f7