bufferpool2.0: Implement buffer invalidation

Change-Id: If7a4a38004f50b4d43a2fae4781f541fe322c249
diff --git a/media/bufferpool/2.0/Android.bp b/media/bufferpool/2.0/Android.bp
index 413125a..cd4e06e 100644
--- a/media/bufferpool/2.0/Android.bp
+++ b/media/bufferpool/2.0/Android.bp
@@ -8,6 +8,7 @@
         "BufferStatus.cpp",
         "ClientManager.cpp",
         "Connection.cpp",
+        "Observer.cpp",
     ],
     export_include_dirs: [
         "include",