It is currently not easy to have a global statusline function, that knows about if the window it is handling (called for) is the active one.
A workaround is tracking WinEnter/WinLeave (and BufWinEnter) events, and setting up a new function for windows then, but that is clumsy.

It would be much easier, if there was a special variable (e.g. v:statusline_winid) that would provide the winid of the window this statusline is for, allowing to just do let active = winid() == v:statusline_winid then.

Note that using an expression has limitations (e.g. #2829), and does not allow for returning highlight groups (%#Foo# - which get used as text).

(There is g:actual_curbuf already, but it is only available for expressions (#3299) (and also not useful when you want to know if the window is the active one).)