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/fuzz/Android.bp b/logd/fuzz/Android.bp
index 299242d..f65fbdf 100644
--- a/logd/fuzz/Android.bp
+++ b/logd/fuzz/Android.bp
@@ -25,6 +25,7 @@
         "liblog",
         "liblogd",
         "libcutils",
+        "libsysutils",
     ],
     cflags: ["-Werror"],
 }