logd: remove SocketClient from LogBuffer and LogBufferElement

In the future, we'll want to be able to write to outputs that are not
necessarily a libsysutils SocketClient, for example host tests of
LogBuffer.  Therefore, we add a LogWriter class to be used instead of
SocketClient.

Test: logging unit tests
Change-Id: I4385be65e14e83a635691a7ba79e9bf060e49484
diff --git a/logd/LogKlog.h b/logd/LogKlog.h
index 77b24bc..56e0452 100644
--- a/logd/LogKlog.h
+++ b/logd/LogKlog.h
@@ -19,10 +19,9 @@
 #include <private/android_logger.h>
 #include <sysutils/SocketListener.h>
 
+#include "LogBuffer.h"
 #include "LogStatistics.h"
 
-class LogBuffer;
-
 class LogKlog : public SocketListener {
     LogBuffer* logbuf;
     const log_time signature;