Include the Postinstall delegate in Android.

The UpdateAttempter in Android was missing the delegate setup for the
postinstall action, so the status updates from the postinstall script
were not showing up in the Binder interface.

Bug: 29223204
TEST=deployed an A/B update to an Android device with a postinstall script.

(cherry picked from commit b6eef73e20736b4b6a43611fa732d8bba72ff7ff)

Change-Id: I0bf421e2b8367c9672a89a872999a0c26b418293
diff --git a/update_attempter_android.cc b/update_attempter_android.cc
index 26d35a4..7b277d7 100644
--- a/update_attempter_android.cc
+++ b/update_attempter_android.cc
@@ -412,6 +412,7 @@
 
   download_action->set_delegate(this);
   download_action_ = download_action;
+  postinstall_runner_action->set_delegate(this);
 
   actions_.push_back(shared_ptr<AbstractAction>(install_plan_action));
   actions_.push_back(shared_ptr<AbstractAction>(download_action));