VolumeDialogImpl: Don't hide the default stream when adjusting the music stream

If there are two visible rows and the user touches the default (music)
row, the other row disappears immediately. Avoid this behaviour by
tracking the row with which this panel was created and keep showing
that row if the user adjusts the music stream.

Test: While receiving a call both rows are touchable and the additional
      row does not disappear when adjusting the music stream.
Change-Id: I4e14a0ea50c5cc41cb279c6fbfc8a7d6e6d1ba61