AudioService: fix A2DP device check when making device unavailable

Fix NPE: when there is no A2DP device connected to APM,
mApmConnectedDevices.get(DEVICE_OUT_BLUETOOTH_A2DP) returns null

Bug: 146466665
Test: turn BT off, turn back on and connect headset
Change-Id: I77d6e58a09aaeaaaacd00e1bf2b7d489ec9ad32d