Remove unnecessary API dependency on Kotlin stdlib.

The user might register with a distributor, and then delete the
distributor app.

If that happens the user is not prompted if there are no distributors,
and re-registration does not happen.

Fix this by checking the ack'ed distributor is still in the list of
installed distributors.

Previous code ignored the dialog's default layout and set its own
textview. I think this was so a LinkMovementMethod could be set on the
message.

This meant the view didn't follow normal Android dialog dimensions
(e.g., padding, margins, text layout), and looked out of place.

Fix this. Use the normal AlertDialog message. Set an onShowListener
that sets the LinkMovementMethod when the dialog is shown.

The strings used in default dialogs were hardcoded. This meant they
couldn't be translated, or use the default platform strings (e.g.,
android.R.string.ok).

Fix that. Convert `RegistrationDialogContent`, `NoDistributorDialog`,
and `ChooseDialog` to interfaces, and provide implementations of those
interfaces that