Add unbind() binder API.

It will remove the callback from the list and unregister death
notification for the callback.

Bug: 62293007
Test: mma
Change-Id: I527699a160feaad739c50ed1865d557d3cef22e8
diff --git a/binder_bindings/android/os/IUpdateEngine.aidl b/binder_bindings/android/os/IUpdateEngine.aidl
index 67f828a..7e26752 100644
--- a/binder_bindings/android/os/IUpdateEngine.aidl
+++ b/binder_bindings/android/os/IUpdateEngine.aidl
@@ -28,6 +28,8 @@
   /** @hide */
   boolean bind(IUpdateEngineCallback callback);
   /** @hide */
+  boolean unbind(IUpdateEngineCallback callback);
+  /** @hide */
   void suspend();
   /** @hide */
   void resume();