Remove A2DP Source/Sink startup calls during A2DP initialization

Don't call btif_a2dp_source_startup() / btif_a2dp_sink_startup()
inside BtifAvSource::Init() / BtifAvSink::Init() .
Those calls are already happening at the right time later during
BtifAvSource::SetActivePeer() / BtifAvSink::SetActivePeer()

Bug: 72701090
Test: Manual with multiple headsets
Change-Id: I634a2c90a7758891565c97e86b83335608ffab69