Force engine initialization during onCreate

- During stress test on browser we noticed that
  registerActivityLifecycleCallbacks.onActivityCreated was not
  getting called. This call was required to ensure that the engine was
  initialized before launching an activity.Added code to ensure engine is
  initialized in onCreate of activity that use engine API's directly.

- Added null check for ComboView.hide method
diff --git a/src/com/android/browser/PhoneUi.java b/src/com/android/browser/PhoneUi.java
index dd18c19..e629952 100644
--- a/src/com/android/browser/PhoneUi.java
+++ b/src/com/android/browser/PhoneUi.java
@@ -114,7 +114,8 @@
 
     @Override
     public void hideComboView() {
-        mComboView.hideViews();
+        if (mComboView != null)
+            mComboView.hideViews();
     }
 
     @Override