Don't add input devices to the list of selected devices.

The Output Switcher has logic that determines the UI behavior based on
the count of selected devices. For example, a group playback UI is
rendered when there are more than one **output** media devices selected.
Adding **input** devices to the list breaks this logic.

Bug: 422288043
Test: atest MediaSwitchingControllerTest
Flag: com.android.media.flags.enable_audio_input_device_routing_and_volume_control
Change-Id: Id5eeb8b26827bb8de41e61e54ae1b7f3143c64b4