liblog: is loggable is flaky

- Deal with __android_log_is_loggable inside signal handler by
  treating a lock contention system call as more costly than reading
  the associated property directly. Rather waiting around in a
  contended stat to hit cache.
- Check both the individual known __system_property_serial() and
  global __system_property_area_serial() to detect updates or
  additions to the properties to respond in the most aggressive and
  timely manner. __android_log_is_loggable() return at max CPU
  clockrate on a N9 in 61ns.
- Craft a common do_cache2 inline that adds the above to the other
  functions that utilize cache handling and preserves (within 3ns)
  performance in android_log_clockid() and __android_log_security().
  These functions return at max CPU clockrate on a N9 in 23ns.

Bug: 19544788
Bug: 25693940
Bug: 25792367
Bug: 26178938
Change-Id: I9cd94598f5c558e946b93977ad3714a4b03d0422