Fix grouping of selected devices to a single control.

With ag/33427211 the `MediaSwitchingController#getSelectedMediaDevice`
checks for selected routes within the list of available routes rather
than making a separate API call. So the method should not be called
prior to receiving routes from the MediaRouter API.

Fix: 418248584
Flag: EXEMPT low risk UI bugfix
Test: atest SystemUiRoboTests:MediaOutputAdapterTest,
manually on a device.

Change-Id: Ie0562a3d3f10a71164a5896500dba37bf12f789c