packages/SystemUI/res/drawable/volume_row_seekbar_progress.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
  ~ Copyright (C) 2021 The Android Open Source Project
  ~
  ~ Licensed under the Apache License, Version 2.0 (the "License");
  ~ you may not use this file except in compliance with the License.
  ~ You may obtain a copy of the License at
  ~
  ~      http://www.apache.org/licenses/LICENSE-2.0
  ~
  ~ Unless required by applicable law or agreed to in writing, software
  ~ distributed under the License is distributed on an "AS IS" BASIS,
  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  ~ See the License for the specific language governing permissions and
  ~ limitations under the License.
  -->

<!-- Progress drawable for volume row SeekBars. This is the accent-colored round rect that moves up
     and down as the progress value changes. -->
<layer-list xmlns:android="http://schemas.android.com/apk/res/android"
    android:autoMirrored="true">
    <item android:id="@+id/volume_seekbar_progress_solid">
        <shape>
            <size android:height="@dimen/volume_dialog_panel_width" />
            <solid android:color="?android:attr/colorAccent" />
            <corners android:radius="@dimen/volume_dialog_panel_width_half"/>
        </shape>
    </item>
    <item
        android:id="@+id/volume_seekbar_progress_icon"
        android:gravity="center_vertical|right"
        android:height="@dimen/rounded_slider_icon_size"
        android:width="@dimen/rounded_slider_icon_size"
        android:right="@dimen/rounded_slider_icon_inset">
        <rotate
            android:fromDegrees="-270"
            android:toDegrees="-270">
            <!-- A placeholder drawable is required here - it'll be replaced in code. -->
            <com.android.systemui.util.AlphaTintDrawableWrapper
                android:drawable="@drawable/ic_volume_media"
                android:tint="?android:attr/colorBackgroundFloating" />
        </rotate>
    </item>
</layer-list>