bluetooth: Replace "Not In Use" status string with "On"

If Bluetooth is on but there aren't connected devices, the status in the
menu reads "Not in Use". This is potentially confusing: it's a negative
statement, even though Bluetooth is on. It also sits uneasily (and looks
even more confusing) next to the submenu item "Turn Off".

Changing the string to "On" is better.

https://bugzilla.gnome.org/show_bug.cgi?id=756432