Fix crash on Storage and Privacy page

The implementation in PreferenceGroup is changed,
contentEquals doesn't accept null value.

So, we won't call findPreference when key of controller
is null.

Test: robotest, maunal test
Fixes: 122805831

Change-Id: Ia6df5c536ca5a32448f66b5308e7ec2334f1fe7b
(cherry picked from commit 3314cd05d5b97368e7c8f7ea7470f7321fbcdf95)

src/com/android/settings/dashboard/DashboardFragment.java

@@ -369,6 +369,11 @@ public abstract class DashboardFragment extends SettingsPreferenceFragment
                 mPreferenceControllers.values()) {
             for (AbstractPreferenceController controller : controllerList) {
                 final String key = controller.getPreferenceKey();
+                if(TextUtils.isEmpty(key)) {
+                    Log.d(TAG, String.format("Preference key is %s in Controller %s",
+                            key, controller.getClass().getSimpleName()));
+                    continue;
+                }
                 final Preference preference = screen.findPreference(key);
                 if (preference != null) {
                     preference.setVisible(visible && controller.isAvailable());