Fix weird ripple circle in the volume SeekBar.

The background is provided by the progressDrawable's background layer, so we can set the regular bg to null to hide the ripple.

Fixes: 188251902
Test: no more weird ripple
Change-Id: I9fee9cb66cb78e50f82b03a292fddcb2a0c40e88

packages/SystemUI/res/layout/volume_dialog_row.xml

@@ -57,6 +57,7 @@
                 android:thumb="@null"
                 android:splitTrack="false"
                 android:progressDrawable="@drawable/volume_row_seekbar"
+                android:background="@null"
                 android:layoutDirection="ltr"
                 android:rotation="270" />
         </FrameLayout>