Support stats log in lmkd.

This CL changes the relevant cpp files to c files. And added functionalities to reset the log
context for reuse.

Background:
+ lmkd doesn't use the generated statslog code because:
   1. lmkd doesn't want to create a 4K log context for each log. lmkd is single threaded and can
      reuse the same log context.
   2. lmkd is written in c, and the existing statslog.h/.cpp are not compatible

Bug: 78603347
Test: manually tested with alloc-stress
Change-Id: Ife6f5c69248ecf5af730269e67f229ba4c72f37f