InCallUI/res/drawable/btn_add.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,
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  ~ See the License for the specific language governing permissions and
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  -->

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:id="@+id/backgroundItem"
          android:drawable="@drawable/btn_background" />

    <item>
        <bitmap android:src="@drawable/ic_toolbar_add_call"
            android:gravity="center"
            android:tint="@color/selectable_icon_tint" />
    </item>

</layer-list>