logd: rename mOldest -> oldest_

I added mOldest recently before mentally committing to have new code
follow the Google C++ style guide.

Test: build
Change-Id: I6d5bab5833e14ac3808862598a2a60989d805e18
diff --git a/logd/LogBuffer.h b/logd/LogBuffer.h
index cf93c23..9a36712 100644
--- a/logd/LogBuffer.h
+++ b/logd/LogBuffer.h
@@ -80,9 +80,6 @@
 
     LogStatistics stats;
 
-    // Keeps track of the iterator to the oldest log message of a given log type, as an
-    // optimization when pruning logs.  Use GetOldest() to retrieve.
-    std::optional<LogBufferElementCollection::iterator> mOldest[LOG_ID_MAX];
     // watermark of any worst/chatty uid processing
     typedef std::unordered_map<uid_t, LogBufferElementCollection::iterator>
         LogBufferIteratorMap;
@@ -167,4 +164,8 @@
 
     LogTags* tags_;
     PruneList* prune_;
+
+    // Keeps track of the iterator to the oldest log message of a given log type, as an
+    // optimization when pruning logs.  Use GetOldest() to retrieve.
+    std::optional<LogBufferElementCollection::iterator> oldest_[LOG_ID_MAX];
 };