Use current_idx api call

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>

subiquity/controllers/__init__.py

@@ -64,7 +64,7 @@ class BaseController:
         raise urwid.ExitMainLoop()
 
     def header_hotkeys(self, key):
-        if key in ['esc'] and Routes.current_route_idx != 0:
+        if key in ['esc'] and Routes.current_idx() != 0:
             Routes.prev()
             current_route = Routes.current()
             current_route(self).show()