lmkd: Support variable polling intervals set by event handlers

After a memory event happens event handler can assess current memory
condition and decide if and when lmkd should re-check memory metrics in
order to respond to changing memory conditions. Change the event handler
interface to allow control over polling period and ability to start/extend
polling session.

Bug: 132642304
Test: lmkd_unit_test
Change-Id: Ia74011e943140b6cffbf452ff8e1744b7336eacf
Signed-off-by: Suren Baghdasaryan <surenb@google.com>