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
diff --git a/liblog/tests/Android.mk b/liblog/tests/Android.mk
index d75bbc9..a407c50 100644
--- a/liblog/tests/Android.mk
+++ b/liblog/tests/Android.mk
@@ -77,6 +77,6 @@
 LOCAL_MODULE := $(test_module_prefix)unit-tests
 LOCAL_MODULE_TAGS := $(test_tags)
 LOCAL_CFLAGS += $(test_c_flags)
-LOCAL_SHARED_LIBRARIES := liblog
+LOCAL_SHARED_LIBRARIES := liblog libcutils
 LOCAL_SRC_FILES := $(test_src_files)
 include $(BUILD_NATIVE_TEST)