liblog: gate write on log id available

- Secure LOG_ID_KERNEL in writer
- Secure LOG_ID_SECURITY in reader and writer
- if writer transport says not available, do not write to that log id

Bug: 27566046
Bug: 27896341
Change-Id: If63a78a56fb94adfbf9979454c4cadb81af45c19
diff --git a/liblog/logger.h b/liblog/logger.h
index 61bc396..5d031d7 100644
--- a/liblog/logger.h
+++ b/liblog/logger.h
@@ -40,6 +40,7 @@
 struct android_log_transport_write {
   struct listnode node;
   const char *name;
+  unsigned logMask; /* cache of available success */
   union android_log_context context; /* Initialized by static allocation */
 
   int (*available)(log_id_t logId);