Add facility to run setup wizard after an update.

The system now keeps a tag of the last version (just an arbitrary string)
that the setup wizard was run for.  If this is different than the current
one in the setup wizard, then setup is launched at boot.

This introduces a new intent action for the part of the setup wizard that
gets run for an ungrade, which the system uses to find its current version
tag for comparing against what was last stored.  It is up to the launched
setup activity update the stored setting to reflect its current value,
once it is happy.