logd: serialize accesses to stats helpers

Quick low-risk to resolve possible hash table corruption.
Resolved an unlikely path memory leak.

ToDo: replace lock with nested lock so no lock
      helpers are required.

Bug: 22068332
Change-Id: I303ab06608502c7d61d42f111a9c43366f184d0c
diff --git a/logd/LogStatistics.h b/logd/LogStatistics.h
index b9e9650..760d6b2 100644
--- a/logd/LogStatistics.h
+++ b/logd/LogStatistics.h
@@ -334,7 +334,7 @@
     // *strp = malloc, balance with free
     void format(char **strp, uid_t uid, unsigned int logMask);
 
-    // helper
+    // helper (must be locked directly or implicitly by mLogElementsLock)
     char *pidToName(pid_t pid);
     uid_t pidToUid(pid_t pid);
     char *uidToName(uid_t uid);