SystemUI: VolumeDialogImpl: Show output button if there is an active stream

The media output dialog can be used to connect different devices.
This patch makes it show up if at least one of the following
conditions is met:
1: There is an active local media controller
2: There is a bluetooth a2dp device connected

Change-Id: Ie2a2a0f3125e1ffbf78bfc1f66868696f6389294