Previously, we retried based on a count. Now we've added the ability to
keep retrying for a specified time, using exponential backoff to
throttle attempts.
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.
1) Remove all our PDU code and switch to the PDU code from the
klinker library
2) Switch to using the system Lollipop MMS library by default,
and falling back to our own custom library if that fails.
3) Format SMIL differently, using code from klinker instead of
what we've pieced together.
4) Pull per-carrier MMS media constraints from the XML config
files in the klinker library, instead of hardcoding it at 280kb.
Hopefully this is an improvement, but given that MMS is involved,
it will probably make things worse instead.
