appinfo: Do not override system default handler with less specific one

appinfo: Do not override system default handler with less specific one

We were considering explicitly configured defaults for parent types
after we already got results for the specific type we're interested in.

This resulted in the explicit default for text/plain to override all
system defaults for subtypes of text/plain, for example. The explicit
default should not apply to subtypes that have a system default.

https://bugzilla.gnome.org/show_bug.cgi?id=642797