Consolidate selected device state in InputRouteManager

- Replace `mSelectedInputDeviceType` and `mSelectedInputDeviceAddr` with
`mSelectedDeviceAttributes`.
- Add a comment that describes the selected device check.

Bug: 432652209
Test: atest InputRouteManagerTest
Flag: com.android.media.flags.enable_audio_input_device_routing_and_volume_control
Change-Id: Id42e90a2fedcd32f960b1cb5ba91336f8eaa2386