Fix NPE in Wi-Fi Direct Setting UI

After a PreferenceGroup is created, attach it to a PreferenceScreen right away.
Otherwise, if PreferenceGroup.addPreference is accidentally called before being
attached, it will result in an NPE and a UI crash.

Bug: 11050625
Change-Id: I68fa8b67ab807c0729c43c1b5cdabdbcd4f2c230

src/com/android/settings/wifi/p2p/WifiP2pSettings.java

@@ -305,9 +305,11 @@ public class WifiP2pSettings extends SettingsPreferenceFragment
 
         mPeersGroup = new PreferenceCategory(getActivity());
         mPeersGroup.setTitle(R.string.wifi_p2p_peer_devices);
+        preferenceScreen.addPreference(mPeersGroup);
 
         mPersistentGroup = new PreferenceCategory(getActivity());
         mPersistentGroup.setTitle(R.string.wifi_p2p_remembered_groups);
+        preferenceScreen.addPreference(mPersistentGroup);
 
         super.onActivityCreated(savedInstanceState);
     }