lmkd: subscribe to low mempressure events.

This will make sure we get invoked in time,
and combined with a kernel change to increase
the window size (and thus rate-limit the
notifications) based on the amount of memory
on the device, this should provide a nice
balance between receiving too many notifications
vs getting notified too late.

Change-Id: I2f5e92390b94e582aaa06d88763a232e39673496