lmkd: move sundry pieces to statslog.h

Pragma once is not part of the standard, and is actually a gnu
C++ addition. Android coding standard requires the #ifndef header
wrappers. Moved things that belong in statslog.h from the lmkd files.

SideEffects: None
Test: lmkd_unit_tests
Bug: 33808187
Bug: 72838192
Change-Id: I9686b1a0791ee2b723d05b91905eda0bb64a1156