lmkd: add logic to kill the heaviest of the eligible processes

Killing the most memory-demanding process from the set of eligible
processes yields better results on high-performance devices than
killing the first one we could find. This is in line with how in-kernel
lowmemorykiller driver chooses its victims.

Bug: 63631020
Test: alloc-stress

Change-Id: Ie1ef7f33f3e79698a9b4120c14490386d6129f9b
Signed-off-by: Suren Baghdasaryan <surenb@google.com>