liblog: add __android_log_is_loggable()

- Add new liblog API __android_log_is_loggable(prio, tag, def)
- future plan to integrate this into the runtime checks and into
  the logd daemon for filtration. Inert for now.

Bug: 17760225
Change-Id: I16395b4d42acc08f0209f55a1cbf87b0b2112898
diff --git a/liblog/Android.mk b/liblog/Android.mk
index a4e5f5e..5756c54 100644
--- a/liblog/Android.mk
+++ b/liblog/Android.mk
@@ -46,7 +46,7 @@
 endif
 
 liblog_host_sources := $(liblog_sources) fake_log_device.c
-liblog_target_sources := $(liblog_sources) log_time.cpp
+liblog_target_sources := $(liblog_sources) log_time.cpp log_is_loggable.c
 ifneq ($(TARGET_USES_LOGD),false)
 liblog_target_sources += log_read.c
 else