SystemUI: VolumeDialogImpl: Move the MediaOutputDialog above status bar

The volume dialog is above the status bar too and can be used while
the statusbar is expanded. Opening the media output dialog then will
cause the dialog to appear below the status bar which isn't visible
until the status bar is collapsed. This change moves the media output
dialog above the status bar.

Change-Id: Ie906ee0a59d52dbb6b6f703e210fa43308d069f0