logd: separate LogStatistics from LogBuffer

LogStatistics is intertwined with LogBuffer, even relying on it for
thread safety.  This needs to change to have a proper
LogBufferInterface, so this CL separates them.  Specifically:

1) Adding a lock to LogStatistics and adding thread annotations to
   ensure that data structures are protected appropriately.
2) Moving prune_rows calculation into LogStatistics so it is done
   while holding this lock.
3) Using LogStatistics instead of LogBuffer where appropriate.

Note that there should not be a significant performance regression
with this lock, as it will almost always been uncontended.  If
anything, it should alleviate pressure from LogBuffer's lock.

Test: logging unit tests
Change-Id: I9d6dde2c96c9f024fa0341711c7bc63379e8e406
diff --git a/logd/LogBufferElement.h b/logd/LogBufferElement.h
index 434b7db..8676cf1 100644
--- a/logd/LogBufferElement.h
+++ b/logd/LogBufferElement.h
@@ -25,6 +25,7 @@
 #include <sysutils/SocketClient.h>
 
 class LogBuffer;
+class LogStatistics;
 
 #define EXPIRE_HOUR_THRESHOLD 24  // Only expire chatty UID logs to preserve
                                   // non-chatty UIDs less than this age in hours
@@ -55,10 +56,9 @@
     static atomic_int_fast64_t sequence;
 
     // assumption: mDropped == true
-    size_t populateDroppedMessage(char*& buffer, LogBuffer* parent,
-                                  bool lastSame);
+    size_t populateDroppedMessage(char*& buffer, LogStatistics* parent, bool lastSame);
 
-   public:
+  public:
     LogBufferElement(log_id_t log_id, log_time realtime, uid_t uid, pid_t pid,
                      pid_t tid, const char* msg, uint16_t len);
     LogBufferElement(const LogBufferElement& elem);
@@ -98,5 +98,5 @@
     }
 
     static const uint64_t FLUSH_ERROR;
-    uint64_t flushTo(SocketClient* writer, LogBuffer* parent, bool lastSame);
+    uint64_t flushTo(SocketClient* writer, LogStatistics* parent, bool lastSame);
 };