logd: create SimpleLogBuffer and implement ChattyLogBuffer in terms of it

Test: unit tests with SimpleLogBuffer
Change-Id: If6e29418645b5491df9b8aeef8f95bb786aeba93
diff --git a/logd/Android.bp b/logd/Android.bp
index 197fc28..1f6ab34 100644
--- a/logd/Android.bp
+++ b/logd/Android.bp
@@ -54,6 +54,7 @@
         "LogStatistics.cpp",
         "LogWhiteBlackList.cpp",
         "LogTags.cpp",
+        "SimpleLogBuffer.cpp",
     ],
     logtags: ["event.logtags"],