mpris: Validate received data against the expected types from the spec

mpris: Validate received data against the expected types from the spec

In the wild we have buggy clients (notably Chromium 77 and earlier) that
send metadata with the wrong types. Previously, this would throw an
exception and prevent the MPRIS information from showing up in the
message list.

This changes the code to check if any incoming metadata is of the type
it is expected to be, and logs a warning if not, then continues on with
a default value.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1362