applet: make applet_find_active_connection_for_device() static

It is only used in applet.c.
applet: fix nm-applet tooltip when a VPN connection is active (bgo #737585)

Active VPN connections contain a device of a base connection, but we are
interested in the base active connection itself.

https://bugzilla.gnome.org/show_bug.cgi?id=737585
build: fix compilation of gnome-bluetooth

Broken by 1bb881d486ebb5500961345a104d7a400074b8e2.