Don't back up / restore certain sync-related settings

In particular, this no longer attempts to back up the on/off state of specific
backend syncing [gmail/contacts/calendar], nor the "background data" toggle.
The former was causing a great deal of spurious trips through backup as the
notification was being tickled during general sync operation, and the latter
makes little sense at restore time.

Fixes these issues:

b/2097613 - frequent "backup_data_changed" messages in event log
b/2131662 - should not backup background data, master sync settings