Fix alert dialogs in VpnSettings.

+ Don't check mDialogFragment.isVisible() in SettingsPreferenceFragment.removeDialog()
  as mDialogFragment may not be visible in parent fragment's onResume().
+ Replace mConnectDialog with mConnectDialogShowing and remove
  removeConnectDialog().
+ Dismiss alert dialogs in onPause() so that we don't need to maintain extra
  states during pause-resume cycle.

In addition, fix a NPE when startVpnTypeSelection().

Bug: 3381434
Bug: 3289365

Change-Id: Ic4aa87c7a618d95e86e45d6617f2ad7dab35f019

src/com/android/settings/SettingsPreferenceFragment.java

@@ -105,8 +105,10 @@ public class SettingsPreferenceFragment extends PreferenceFragment
     }
 
     protected void removeDialog(int dialogId) {
-        if (mDialogFragment != null && mDialogFragment.getDialogId() == dialogId
-                && mDialogFragment.isVisible()) {
+        // mDialogFragment may not be visible yet in parent fragment's onResume().
+        // To be able to dismiss dialog at that time, don't check
+        // mDialogFragment.isVisible().
+        if (mDialogFragment != null && mDialogFragment.getDialogId() == dialogId) {
             mDialogFragment.dismiss();
         }
         mDialogFragment = null;