AVRCP: Change notification variable when the interim response is sent

This avoids the possibility of having a changed notification be sent
before an interim response is sent when the AVRCP service has to wait
for a response from the media layer for information regarding the
notification.

Also fix the PlayPosition test for AVRCP as it was testing for play
state correctness.

Bug: 109588762
Test: Run host test net_test_avrcp
Change-Id: Icb7bb1068191ecf2a2e390854084b6b9b47537fa