Add bind() function to the binder interface.

Update engine may have started and done some work before we call
applyPayload(). Add a new binder function bind() to register a
callback function that communicates with the update engine.

Bug: None
Test: 'mmma system/update_engine'.
Change-Id: Iafcdb250904b44f9526be15f0381690df50805a2
diff --git a/binder_bindings/android/os/IUpdateEngine.aidl b/binder_bindings/android/os/IUpdateEngine.aidl
index ebc3ffb..1e8090e 100644
--- a/binder_bindings/android/os/IUpdateEngine.aidl
+++ b/binder_bindings/android/os/IUpdateEngine.aidl
@@ -16,12 +16,12 @@
 
 package android.os;
 
-import android.os.IUpdateEnginePayloadApplicationCallback;
+import android.os.IUpdateEngineCallback;
 
 interface IUpdateEngine {
-  int applyPayload(String url,
-                   in String[] headerKeyValuePairs,
-                   IUpdateEnginePayloadApplicationCallback callback);
+  void applyPayload(String url,
+                    in String[] headerKeyValuePairs);
+  boolean bind(IUpdateEngineCallback callback);
   void suspend();
   void resume();
   void cancel();