Clear the device's data from the transport when backup is disabled

Turning off backup in the Settings UI constitutes an opt-out of the whole
mechanism.  For privacy reasons we instruct the backend to wipe all of the data
belonging to this device when the user does this.  If the attempt fails it is
rescheduled in the future based on the transport's requestBackupTime()
suggestion.  If network connectivity changes prompt the transport to indicate a
backup pass is appropriate "now," any pending init operation is processed before
the backup schedule is resumed.

The broadcasts used internally to the backup manager are now fully protected;
third party apps can neither send nor receive them.

(Also a minor logging change; don't log 'appropriate' EOF encountered during
parsing of a backup data stream.)