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.

Change-Id: Ifd086704d1161f348bf0dd405a6a9d9cb8b69990
diff --git a/update_attempter_android.cc b/update_attempter_android.cc
index 2e7fda6..30593dc 100644
--- a/update_attempter_android.cc
+++ b/update_attempter_android.cc
@@ -411,6 +411,7 @@
 
   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));