Allow user to email exported APKG

It can be a bit of a pain to use USB to copy an exported APKG, so I got AnkiDroid to ask if the user wants to email the apkg at the end of the export. This was motivated by [this thread](https://groups.google.com/d/msg/anki-android/oAbdlNhiNyk/SyI7WSQN-0cJ) on the forum, but it should also be useful in many other places, and also for us when providing support.