gnome: rework the switch source functions

- Refactor the logic inside the getter functions for the input source
  functions.
- Use the glib everywhere.
- Remove some memory leaks.
- Add a sflphone_toggle_camera() function to switch between the
  active_device and the none.

Change-Id: Iae83c5b4922722c8914ba9339c6d53b8e9ef53d8
Refs: #45417