Add back private PlayStoreUtil constructor.

It was taken out in a public PR, thought I added it back in, but
apparently didn't.

app/src/main/java/org/thoughtcrime/securesms/util/PlayStoreUtil.java

@@ -11,6 +11,8 @@ import org.thoughtcrime.securesms.BuildConfig;
 
 public final class PlayStoreUtil {
 
+  private PlayStoreUtil() {}
+
   public static void openPlayStoreOrOurApkDownloadPage(@NonNull Context context) {
     if (BuildConfig.PLAY_STORE_DISABLED) {
       CommunicationActions.openBrowserLink(context, "https://signal.org/android/apk");