avrcp: fix responding to rejected volume notification registration

According to the AVRCP specification v1.6.2 section 6.15.1, When a
register notification request is rejected, the rejecting packet needs
to have an error code field instead of the event field. Therefore, we
shouldn't check the event field for those rejection packets.

Bug: 356551220
Test: Pair with Logitech Z207. Verify AVRCP connection is reported.
Flag: EXEMPT, bugfix

Change-Id: Idd515de2ac6d6cf74d9a6bd5628773f43171175a