transport-xmpp: Update ServerPingWithAlarmManager

- fix typo: s/onDestory/onDestroy/
- mEnabled should be non-static
- add javadoc
- add XMPPConnectionRegistry entry
transport-xmpp: Use non-constant reconnect delay

I'm not 100% how the reconnect calculation is done with this this
change. At first it's a constant delay of 10 seconds for the first 60
retries (=10min). Then we use basically

10 seconds * (<reconnectionAttemptCountAfterConstant>^1.2)

and cap the maximum delay by 30 minutes.

Ideal we would use a function that starts with small increases, but
grows larger after some time.
transport-xmpp: Switch to Smack 4.1.0-alpha2