lmkd: Switch to using /proc/meminfo to have access to file cache size

Current mechanism of getting system memory state by using sysinfo()
is not enough because it does not provide information about file cache
size which is needed to correctly assess memory state. Switch to using
data from /proc/meminfo that includes information provided by sysinfo()
and file cache size in addition to that.

Bug: 77299493
Bug: 75322373
Change-Id: I16106fc4f9254f17f776803e60502b7b6474e1b7
Signed-off-by: Suren Baghdasaryan <surenb@google.com>