Fix tracking of backup participants across package remove/update

                int uid = app.uid;

                HashSet<ApplicationInfo> set = mBackupParticipants.get(uid);

                if (set != null) {

                    set.remove(app);

                    // Find the existing entry with the same package name, and remove it.

                    // We can't just remove(app) because the instances are different.

                    for (ApplicationInfo entry: set) {

                        if (entry.packageName.equals(app.packageName)) {

                            set.remove(entry);

                            break;

                        }

                    }

                    if (set.size() == 0) {

                        mBackupParticipants.delete(uid);                    }

                }