Populate `InputMediaDevice#isSelected`.

The UI relies on `isSelected` attribute to make the device in the list
interactive (has volume control).

`InputMediaDevice#isSelected` replaces
`InputRouteManager#getSelectedDevice()` which usage was removed in
ag/34630947.

Fix: 432652209
Test: atest InputRouteManagerTest InputMediaDeviceTest
Test: atest MediaOutputAdapterTest MediaSwitchingControllerTest
Flag: com.android.media.flags.enable_audio_input_device_routing_and_volume_control
Change-Id: I683db6ed09f5038d2671b3ce3e931b22f1e098b3