Handle incoming calls on lock screen

This is builds on top of !777 (merged) since @tobias.bernard is reworking the 'incoming call when locked' case and this way we have already have most of the code skeleton in place for that. No need to review yet.

TODO:

• wire up avatar and name label
• wire up callaudiod
• TODOs in source
• add encryption indicator
• adjust to mockups
• disable dialpad or allow DMTF

Closes: calls#107

Relevant mockups: https://gitlab.gnome.org/Teams/Design/os-mockups/-/blob/master/mobile-shell/lockscreen-calls.png

we'll need to check how close we want to come for an initial merge since calls#107 is worth fixing on its own so e.g. i'd do the swipeable buttons as follow up.