Previous active volume dialog remains above a11y.

When the volume dialog is unexpanded, the a11y volume
slider displays below the current active volume stream. Previously,
when adjusting a11y volume, the previous active stream's volume dialog
row goes out of view once a11y becomes the active stream.
This caused the a11y volume dialog row to display strangely.
Now, the previous active volume dialog remains above the a11y volume
dialog when adjusting a11y volume.

Change-Id: Ice09a5c52dc524956bc5acf33f27bfd4a9843400
Fixes: 64326406
Test: manual