Fix crash in ChooseLockPassworkFragment.

Check for null activity when the handler receives messages to ensure
that the fragment is still attached to the activity.

Fixes: 31034901

Test: Manual - Manually try to update password, verify it works. Run
monkey test on settings package and verify no crash seen.

Change-Id: I9753729ee64458fa1bc25d78abc946d2563a5067

src/com/android/settings/ChooseLockPassword.java

@@ -899,6 +899,9 @@ public class ChooseLockPassword extends SettingsActivity {
 
             @Override
             public void handleMessage(Message msg) {
+                if (getActivity() == null) {
+                    return;
+                }
                 if (msg.what == ON_TEXT_CHANGED) {
                     updateUi();
                 }