Only activate device that is not existed previously

Since onAudioDevicesAdded is called not only when new device is hot
plugged, but also when the switcher dialog is opened, adding a check to
make sure only activate if the device does not exist previously.

Change-Id: Icf9756c38ed78be7898e5094f62627025bdb253c
Bug: b/379798051, b/355684672
Test: InputRouteManagerTest
Flag: com.android.media.flags.enable_audio_input_device_routing_and_volume_control