Due to an Android P bug, we basically need to stop calling
startService() in onResume()/onPause(). That means I had to turn
MessageRetrieval service into a singlton instead of a service. I also
moved the offending KeyCachingService calls into static methods that
didn't have to start the service.
We have to make some changes, and it's gotten to the point where
maintaining it as a separate library is more hassle than it's worth,
especially with Google releasing WorkManager as the preferred job
scheduling library.
Eliminate the concept of 'Recipients' (plural). There is now just
a 'Recipient', which contains an Address that is either an individual
or a group ID.
MMS groups now exist as part of the group database, just like push
groups.
// FREEBIE
This was a holdover from Signal's origins as a pure SMS app.
It causes problems, depends on undefined device specific behavior,
and should no longer be necessary now that we have all the
information we need to E164 all numbers.
// FREEBIE
1) Prefetch identity keys when possible
2) Always accept prefetched keys or keys from incoming messages
3) Block sending only if it's a recent change, or if always
block is enabled
// FREEBIE
