Use correct call btif_a2dp_sink_startup() for Sink peers

Replaced an incorrect call to btif_a2dp_source_startup() with
btif_a2dp_sink_startup() inside BtifAvSink::SetActivePeer()

Also, minor cleanup: fix log messages, add missing description for
some of the methods.

Bug: 72701090
Test: Manual with Headet
Change-Id: I0620833b02bd9012fea63ce2ce5e3c4c74d27bf7