[Android Auto] Investigate how we need to handle trusted hosts

Nobody has claimed this yet.

@kmadsen commented on [Thu May 27 2021](https://github.com/mapbox/1tap-android/issues/988)

The car library has a [HostValidator](https://developer.android.com/reference/androidx/car/app/validation/HostValidator)

Which essentially makes the 1tap app a trusted host with the head unit. You can find where this is in the `MainCarAppService`. I don't fully understand either, so this ticket is to figure it out and share your findings.

``` kotlin
class MainCarAppService : CarAppService() {
override fun createHostValidator(): HostValidator {
return HostValidator.ALLOW_ALL_HOSTS_VALIDATOR
// TODO limit hosts for production
// https://github.com/mapbox/1tap-android/issues/988
// return HostValidator.Builder(this)
// .addAllowedHosts(R.array.android_auto_allow_list)
// .build()
}

override fun onCreateSession(): Session {
return MainCarSession()
}
}

```

Start with MainCarAppService and the linked AndroidX HostValidator documentation, focusing on the current ALLOW_ALL_HOSTS_VALIDATOR configuration and the commented allow-list alternative. Document how trusted hosts should be handled for production and share the findings and recommendation in the issue.

Written by the indexing model from the issue text.