lmkd: Change zoneinfo parsing to retrieve zone watermarks

/proc/zoneinfo contains per-node data and each node contains per-zone
section for each zone. Current parser does not recognize this hierarchy
and useful per-zone information like zone watermarks cannot be retrieved.
Change the parser to parse zoneinfo into a hierarchical structure. New
parser also handles up to 2 nodes and can be easily extended to handle
more if needed by changing MAX_NR_NODES.

Bug: 132642304
Test: lmkd_unit_test
Change-Id: I9306289ea6d30d78a261c5d5c29f4f6ea167807d
Signed-off-by: Suren Baghdasaryan <surenb@google.com>