liblog: add log/log_system.h

Move SLOG logging macros to their own home.

SLOGV
SLOGV_IF
SLOGD
SLOGD_IF
SLOGI
SLOGI_IF
SLOGW
SLOGW_IF
SLOGE
SLOGE_IF

Test: gTest liblog-unit-tests --gtest_filter=liblog.SLOG
Bug: 34250038
Change-Id: I456c7ad23152dc1c7b39143d87401a8737e24d64