logd: switch asprintf to std::string

Bug: 23350706
Change-Id: I715cdd4563a09de3680081947a3439f0cac623be
diff --git a/logd/LogBuffer.cpp b/logd/LogBuffer.cpp
index db3f26c..6ea4109 100644
--- a/logd/LogBuffer.cpp
+++ b/logd/LogBuffer.cpp
@@ -690,10 +690,12 @@
     return max;
 }
 
-void LogBuffer::formatStatistics(char **strp, uid_t uid, unsigned int logMask) {
+std::string LogBuffer::formatStatistics(uid_t uid, unsigned int logMask) {
     pthread_mutex_lock(&mLogElementsLock);
 
-    stats.format(strp, uid, logMask);
+    std::string ret = stats.format(uid, logMask);
 
     pthread_mutex_unlock(&mLogElementsLock);
+
+    return ret;
 }