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/include/log/log.h b/include/log/log.h
index 3d86533..3cc2522 100644
--- a/include/log/log.h
+++ b/include/log/log.h
@@ -556,6 +556,12 @@
 #define typeof_log_id_t unsigned char
 
 /*
+ * Use the per-tag properties "log.tag.<tagname>" to generate a runtime
+ * result of non-zero to expose a log.
+ */
+int __android_log_is_loggable(int prio, const char *tag, int def);
+
+/*
  * Send a simple string to the log.
  */
 int __android_log_buf_write(int bufID, int prio, const char *tag, const char *text);