Implement IBufferClient hwbinder interface

Some boiler plate code for future use.

Test: "atest BufferHubBuffer_test" passed.
Bug: b/116681016

Change-Id: I12854ac6f553777451584e86a81f2e6064a12696
diff --git a/services/bufferhub/Android.bp b/services/bufferhub/Android.bp
index d03d833..ca65e02 100644
--- a/services/bufferhub/Android.bp
+++ b/services/bufferhub/Android.bp
@@ -22,6 +22,7 @@
         "-Wextra",
     ],
     srcs: [
+        "BufferClient.cpp",
         "BufferHubService.cpp",
         "BufferNode.cpp",
     ],