liblog: audit declare LIBLOG_ABI_PUBLIC

- replace <sys/cdefs.h> with local "log_cdefs.h" which
  fortifies and expands definitions, adding LIBLOG_ABI_PUBLIC,
  LIBLOG_HIDDEN, LIBLOG_ABI_PRIVATE and LIBLOG_WEAK.
- clearly tag each interface as LIBLOG_ABI_PUBLIC, LIBLOG_HIDDEN,
  LIBLOG_ABI_PRIVATE, LIBLOG_WEAK or static depending on scope
- Add -fvisibility=hidden to ensure nothing else leaks
- some code standard adjustments

Bug: 27566046
Change-Id: Ic14033c4e6d833d973beb035ddc1c6134fb35a3f