gnome_mobile: refactor mobile/desktop constants
Currently a single variable ‘$testapi::form_factor_postfix’ is used to both check for mobile tests and to build the name of the required needle tag. Improve code readability by using a global variable ‘$form_factor’ to distinguish mobile from desktop tests, and then use this variable in a separate function to build the name of the required needle tag.
This is a refactoring change only.
Fixes: #106