Exclude query results located on removed media.

Only include results whose tracker:available property is true, or if
query is for an aggregate type then only include results which aggregate
data from a item with a true tracker:available property.

Change-Id: Iaf7487fb33fa0f7bdc0537ba3240fa4cb2c93a44
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>