Ignore a2dp connection request in StateClosing

Receiving BTIF_AV_CONNECT_REQ_EVT in BtifAvStateMachine:StateClosing
will cause the state machine to get stuck. Add a transition to
StateIdle to release this, connection request will fail though.

Bug: 211415049
Test: atest BluetoothInstrumentationTests
Change-Id: Ic34ac5013a20dcb5ff29f91ec1b19bbaff08dba8