lmkd: Allow killing multiple processes to downgrade memory pressure

Record free memory at low vmpressure levels and whenever pressure
increases beyond low free up enough memory to downgrade memory pressure
to low. This is done by freeing enough memory to get to the max free
memory levels seen during low vmpressure.
The kill logic for Go devices is not changed as these devices are designed
to operate under high memory pressure.

Bug: 63631020
Test: alloc-stress

Change-Id: Ic8396eee08013b1c709072a13525601d5c8bf1f1
Signed-off-by: Suren Baghdasaryan <surenb@google.com>