liblog: __android_log_is_loggable support global properties

- Add support for "log.tag" and "persist.log.tag" global
  logging properties, effectively a runtime default minimum
  logging level.
- Add a thread-safe single level cache plus selective logic for the
  four properties being checked to help speed up logging decision
  on subsequent identical calls.
- Using new __system_property_area_serial() to make for
  efficient (<100ns) handling of cache misses. Despite adding
  two new properties, we are 8 times faster on subsequent calls
  even if the properties do not exist.
- A NULL or blank tag is no longer directed to return default,
  it will check the pair of global logging properties first.
- Add liblog.is_loggable gTest
- Fixup liblog.android_logger_get_, allow no content in crash buffer
- Fixup liblog.max_payload, lowered logd priority increases latency

Bug: 19544788
Bug: 21696721
Change-Id: Ideb887755aa3f1fd14a2603bda1fe23cba49642c