Search filters for get_all_system_metadata should use lists

The search filter should have the form:
[{'foo'}: ['bar'], {'baz'}: ['quux']]

The underlying code accidentally and unfortunately allows filters that look like
[{'foo'}: 'bar', {'baz'}: 'quux'], but that is not guaranteed to work in the future.

Change-Id: Ic28612c8c843194e2f4f6918166177813fd5dee3
Merge "Search filters for get_all_system_metadata should use lists"