Define my app as Google Assistant's default music service

I am developping an Android Music Player application. I'd like to integrate it with Google Assistant to control playback and search songs by voice. I already implemented a Media Session in my app according to instructions from the official documentation.

When requesting the Assistant to play something with my app, I have to say something like "Play some artist on MyApp". When I say "Play some artist", Assistant tells me that I don't have configured a default music provider.

This leads me to the following screen in the Google Assistant app:

I note that MyApp is not listed in "Your music services". My question is the following: what do I have to change in my app for it to be choosable as the default music service ?

For your information, here is the Manifest file:

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="fr.nihilus.music">

    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WAKE_LOCK" />

    <uses-feature
        android:name="android.hardware.audio.output"
        android:required="true" />

    <application
        android:name=".NihilusMusicApplication"
        android:allowBackup="true"
        android:fullBackupContent="@xml/backup"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme"
        tools:ignore="GoogleAppIndexingWarning">

        <!-- Main activity for music browsing. -->
        <activity
            android:name=".HomeActivity"
            android:launchMode="singleTop"
            android:theme="@style/AppTheme.NoActionBar">

            <!-- Make this Activity visible in launcher -->
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>

            <!-- Make this activity available as the default music app. -->
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.APP_MUSIC" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>

            <!-- Intent filter that enables voice searches, such as "Play the Beatles" -->
            <intent-filter>
                <action android:name="android.media.action.MEDIA_PLAY_FROM_SEARCH" />

                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>

            <meta-data
                android:name="android.app.default_searchable"
                android:value=".ui.SearchResultActivity" />
            <meta-data
                android:name="android.app.shortcuts"
                android:resource="@xml/shortcuts" />
        </activity>

        <!-- Activity showing detail and tracks of an album. -->
        <activity
            android:name=".ui.albums.AlbumDetailActivity"
            android:parentActivityName=".HomeActivity"
            android:theme="@style/AppTheme.NoActionBar" />

        <!-- Activity for toggling user preferences. -->
        <activity
            android:name=".settings.SettingsActivity"
            android:label="@string/action_settings"
            android:parentActivityName=".HomeActivity" />

        <!--
            Activity to be opened as a dialog window when a track is selected
            from the file explorer.
         -->
        <activity android:name=".FileViewerActivity"
            android:theme="@style/AppTheme.FileViewer"
            android:excludeFromRecents="true">
            <intent-filter>
                <action android:name="android.intent.action.VIEW" />

                <category android:name="android.intent.category.DEFAULT" />

                <data
                    android:mimeType="audio/*"
                    android:scheme="content" />
            </intent-filter>
        </activity>

        <!--
            Main music service, provides media browsing and media playback services to
            consumers through MediaBrowserService and MediaSession. Consumers connect to it through
            MediaBrowser (for browsing) and MediaController (for play session control)
        -->
        <service
            android:name=".service.MusicService"
            android:exported="true"
            android:label="@string/playback_service_label"
            android:process=":musicplayer"
            tools:ignore="ExportedService">
            <intent-filter>
                <action android:name="android.media.browse.MediaBrowserService" />
            </intent-filter>
        </service>

        <!--
            A receiver that will receive media buttons and send as intents to MusicService.
            Required on pre-Lollipop.
        -->
        <receiver android:name="android.support.v4.media.session.MediaButtonReceiver">
            <intent-filter>
                <action android:name="android.intent.action.MEDIA_BUTTON" />
            </intent-filter>
        </receiver>
    </application>

</manifest>

Google assistant at the moment allows certain partner apps in the list. To partner I'm not sure exactly what you do. If you are at one of the large music provider companies you probably already have a Google Business Development rep who should be able to connect you with the right people.

If you aren't already in touch with Google somehow, there are various contact options on this page: https://developers.google.com/actions/ in the "Get the support you need from day one" section. I'd try on or more of these, which hopefully will get you to the right team.