handle lack of storage access and MTP more sensibly

a device without StorageAccess cannot be mounted, so do not show the
mount default action for it, instead use the file manager action (with
the assumption that the trigger actually knows what to do with it, which
chances are it does not because we ignore the predicate system for some
reason - I've recorded a todo about that)

when then triggering the default action we check if the device supports
MTP and if so run the MTP predicate instead of the openin predicate

the end result is that phones with only MTP actually open via mtp://