lmkd: Introduce system property to get minfree and oom_adj levels

Introduce sys.lmk.minfree_levels system property to allow minfree level
reporting. The format for this property is:

<minfree 1>:<oom_adj 1>, <minfree 2>:<oom_adj 2>, ...

Max number of minfree levels is 6 and they are specified in the
increasing order. For example:

sys.lmk.minfree_levels=18432:0,23040:100,27648:200,32256:300,55296:900,80640:906

sys.lmk.minfree_levels updates are ratelimited to once per second in order
to prevent DoS attacks.

Bug: 111521182
Test: getprop sys.lmk.minfree_levels returns expected value
Change-Id: I80d75d6836650b12457d6a99ca88898535837a97
Signed-off-by: Suren Baghdasaryan <surenb@google.com>