applet: always show symbolic icon when in a panel

The icon used for the tooltip--which is often non-symbolic--is fine for
the applet's CompactRepresentation when it's on the desktop, but not
when it's on a panel.

In the future, connectionIconProvider.connectionTooltipIcon should be
made full-color on a more consistent basis. But it's fine for now.

CCBUG: 473278