logd: correctly label identical lines

Move lastTid array from local in LogBuffer::flushTo to per-reader
context in LogTimes::mLastTid and pass into LogBuffer::flushTo.

Replace NULL with nullptr in touched files.

Simplify LogTimeEntry::cleanSkip_Locked initialization of skipAhead
to memset, to match mLastTid memset initialization.

Test: gTest liblog-unit-tests, logd-unit-tests & logcat-unit-tests
Test: adb logcat -b all | grep chatty | grep -v identical
Bug: 36488201
Change-Id: I0c3887f220a57f80c0490be4b182657b9563aa3f
diff --git a/logd/LogBuffer.h b/logd/LogBuffer.h
index fcf6b9a..19d11cb 100644
--- a/logd/LogBuffer.h
+++ b/logd/LogBuffer.h
@@ -115,11 +115,15 @@
 
     int log(log_id_t log_id, log_time realtime, uid_t uid, pid_t pid, pid_t tid,
             const char* msg, unsigned short len);
+    // lastTid is an optional context to help detect if the last previous
+    // valid message was from the same source so we can differentiate chatty
+    // filter types (identical or expired)
     log_time flushTo(SocketClient* writer, const log_time& start,
+                     pid_t* lastTid,  // &lastTid[LOG_ID_MAX] or nullptr
                      bool privileged, bool security,
                      int (*filter)(const LogBufferElement* element,
-                                   void* arg) = NULL,
-                     void* arg = NULL);
+                                   void* arg) = nullptr,
+                     void* arg = nullptr);
 
     bool clear(log_id_t id, uid_t uid = AID_ROOT);
     unsigned long getSize(log_id_t id);