A2DP: Disconnect AVDTP only when there is a link

bta_av_cb.conn_lcb within AV control block is the index bitmask of all
used LCBs which is shared between all connected links, and is changed
when related AVDTP signal channel up or down. When disconnecting the
AVDTP link, we should only check its bitmask and ignore others.

Bug: 145326274
Bug: 146849922
Test: Connect with multiple A2DP sinks
Change-Id: I49a602dbcd76254d9b40eb7b19ffd16d3b45ba3d