Fix NPE when building suggestion identifier.

And refactored the getIdent logic into feature provider.

Change-Id: Id6f66a6942cbaf6d26ae4dca62037a6cf01179a5
Fix: 36314240
Test: make RunSettingsRoboTests

src/com/android/settings/dashboard/suggestions/SuggestionFeatureProvider.java

@@ -49,4 +49,9 @@ public interface SuggestionFeatureProvider {
      * Dismisses a suggestion.
      */
     void dismissSuggestion(Context context, SuggestionParser parser, Tile suggestion);
+
+    /**
+     * Returns an identifier for the suggestion
+     */
+    String getSuggestionIdentifier(Context context, Tile suggestion);
 }