Recognize both handheld and phone clienttypes as mobile devices

Recognize both handheld and phone clienttypes as mobile devices

Empathy currently displays a phone icon for clients which indicate that
they're phones. However some mobile clients use the "handheld" client
type instead (e.g. Xabber on android devices).

While changing things around, i've also refactored the code a bit to
ensure that the determination will stay consistent in the various
location if it's changed in future.