Merge "[Telephony] Return if context is null when register TelephonyCallback"

     */

    public void registerTelephonyCallback(@NonNull @CallbackExecutor Executor executor,

            @NonNull TelephonyCallback callback) {

        if (mContext == null) {

            throw new IllegalStateException("telephony service is null.");

        }

        if (executor == null || callback == null) {

            throw new IllegalArgumentException("TelephonyCallback and executor must be non-null");

        }