So this appears the issue with Podcini not showing up on car display, from Google document:
> To test your app in real vehicles, it must be installed from a trusted source such as the Play Store, with one exception detailed in [Allow unknown sources](https://developer.android.com/training/cars/testing#unknown-sources).
I know "such as" indicates there are other trusted sources, but I don't know which are deemed as such. Podcini is currently not listed yet on Google Play. To work around it, again from Google document:
> On Android Auto, there is also a [developer option](https://developer.android.com/training/cars/testing#developer-mode) to enable running apps not installed from a trusted source. This setting only applies to [media](https://developer.android.com/training/cars/media) and [messaging](https://developer.android.com/training/cars/messaging) apps
