Re-tune low memory killer params.

Drop the minimums back down to their old values.

Revert what I think was a mistake in bumping up the last two
maximums to the same value as was being forced for 64 bit.

Smarten the 64 bit adjustment to be relative to the values picked,
rather than hard-coded.

Change-Id: Ibee9625073469ad4722a1b6684c9fb2b9f0a4681