InCallUI/res/drawable/btn_background.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
  ~ Copyright (C) 2014 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
  -->

<!-- Background resource for "compound buttons" in the in-call UI.
     These buttons have two states (checked and unchecked), and
     show a blue bar along the bottom edge when checked. -->
<ripple xmlns:android="http://schemas.android.com/apk/res/android"
    android:color="@color/incall_accent_color">
    <item>
        <selector>

            <item android:state_focused="true"
                  android:drawable="@drawable/btn_unselected_focused" />

            <item android:drawable="@drawable/btn_unselected" />

        </selector>
    </item>
</ripple>