lmkd: Optimize frequent file reads by keeping file descriptors open

To check system memory state lmkd is using same files every time vmpressure
event is received. Instead of opening and closing these files every time
we store the file descriptor and use pread() to reread the file from
the beginning.

Bug: 77299493
Bug: 75322373
Change-Id: I8e27f8b9526e82e3cc313a02fce215c2e4dd3d29
Signed-off-by: Suren Baghdasaryan <surenb@google.com>