RECEIVER_EXPORTED is mandatory for receivers when target SDK is
14. However, this flag is only available since API 33. So we need to use
Compat.kt to register broadcast receivers.

By default, receivers were exported, so adding this flag creates no change in behaviour.

I checked whether we could try to not export the receiver for "downloaded decks" broadcast.  The app certainly builds, but the intent is never received. So I assume the webview used to download the deck is considered to be an external app.

The app does not crash on api 24 on simulator. So it seems it's still okay not to use those flags when they are not available. I hope the play store will agree with this logic.

There was a slight problem, with variables that could potentially be null. To ensure the type checker agreed with the code, without using !!, I had to rewrite some of the logic.

I also ensured that some of the code that was copy/pasted in various activity now entirely belong to AnkiActivity. As a side effect, all activities that used to listen to the "unmount" notification and finished, will now also show a toast when the card gets unmounted. Currently it was only the deck picker.

Fixes State whether register are received ankidroid#16713

This does not change the behaviour of the app, but allows to target
sdk 14.

This version of the CL is actually by DavidA

Fixed ankidroid#16713