Clear mDialogFragment when it's detached

and re-associate it when it's re-created.

Before this CL, the association is gone when fragment goes through the pause-resume
cycle.

Similarly, restore onDismiss and onCancel listeners in VpnSettings.onCreateDialog(),
restore states in onCreate() instead of onActivityCreated() so that screen rotation
can be handled correctly.

Now that profiles are shared between Settings instances, always handle state change
in VpnSettings.changeState() so that state changed in one instance can be conveyed
to the other and preferences can be correctly enabled/disabled.

In additions, fix some trivial mistakes in VpnSettings.

Bug: 3396394
Change-Id: I242e1ed6c6d410b4dfefb373d8f98266fc9b46d0

src/com/android/settings/SettingsPreferenceFragment.java

@@ -182,6 +182,8 @@ public class SettingsPreferenceFragment extends PreferenceFragment
                                         + DialogCreatable.class.getName());
                     }
                 }
+                // restore mDialogFragment in mParentFragment
+                ((SettingsPreferenceFragment) mParentFragment).mDialogFragment = this;
             }
             return ((DialogCreatable) mParentFragment).onCreateDialog(mDialogId);
         }
@@ -204,6 +206,16 @@ public class SettingsPreferenceFragment extends PreferenceFragment
         public int getDialogId() {
             return mDialogId;
         }
+
+        @Override
+        public void onDetach() {
+            super.onDetach();
+
+            // in case the dialog is not explicitly removed by removeDialog()
+            if (((SettingsPreferenceFragment) mParentFragment).mDialogFragment == this) {
+                ((SettingsPreferenceFragment) mParentFragment).mDialogFragment = null;
+            }
+        }
     }
 
     protected boolean hasNextButton() {