[applet] Fix tooltip showing wrong name for connected device

This was introduced with c810d1f3
and is my fault, sorry. `device` is undefined in the single device use
case; we need to do `connectedDevices[0]` instead to make sure we get
the right one.

BUG: 422691
FIXED-IN: 5.21.4
(cherry picked from commit 29a535b1)