* Relax chrono specification for AnkiDroid
https://github.com/ankidroid/Anki-Android-Backend/pull/251
* Add AnkiDroid service and AnkiDroid customizations
Most of the work here was done by David in the Backend repo; integrating
it into this repo for ease of future maintenance.
Based on 5d9f262f4c
with some tweaks:
- Protobuf imports have been fixed to match the recent refactor
- FatalError has been renamed to AnkidroidPanicError
- Tweaks to the desktop code to deal with the extra arg to open_collection,
and exclude AnkiDroid service methods from our Python code.
* Refactor AnkiDroid's DB code to avoid uses of unsafe
