logd: make LogBuffer an interface

We may use different implementations of LogBuffer in the future, so we
make it interface and create a concrete ChattyLogBuffer class that
implements it.

Test: logging unit tests
Change-Id: I5731d6404640664c9acc26b7c677dff3110c6a11
diff --git a/logd/Android.bp b/logd/Android.bp
index 4f1b2b2..80e3cb2 100644
--- a/logd/Android.bp
+++ b/logd/Android.bp
@@ -33,12 +33,12 @@
 
     srcs: [
         "LogCommand.cpp",
+        "ChattyLogBuffer.cpp",
         "CommandListener.cpp",
         "LogListener.cpp",
         "LogReader.cpp",
         "LogReaderList.cpp",
         "LogReaderThread.cpp",
-        "LogBuffer.cpp",
         "LogBufferElement.cpp",
         "LogStatistics.cpp",
         "LogWhiteBlackList.cpp",