Backup all backupable settings

We were backing up only settings that are backupable and not
null. However, for some settings null is a valid value meaning
nothing. For example, the currently selected autofill service.
Generally, if the setting is present in the settings database,
we need to backup whatever the value is there as it was set
by either the user or the system explicitly, otherwise the
setting would not be present in the database, hence no attempt
to back it up would be made.

Test: manual

bug:63631627

Change-Id: Iedffd7696fbcea9a2b211c9313b668ff3809a292