Release application Google Maps native android

I am compiling an app that uses google maps in android studio . Your debug works perfectly, but when compiling it does not load the map. I would like to know what mistake I may be having. In both files located in debug and release called google_maps_api where the key is defined I have it defined. Then I do not know what mistake I may be having;

<string name="google_maps_key" templateMergeStrategy="preserve" translatable="true">MICLAVE</string>

My manifest :

<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
 package="com.ponny.radiomobile" >

<!--
     The ACCESS_COARSE/FINE_LOCATION permissions are not required to use
     Google Maps Android API v2, but you must specify either coarse or fine
     location permissions for the 'MyLocation' functionality. 
-->
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

<supports-screens
    android:anyDensity="true"
    android:largeScreens="true"
    android:normalScreens="true"
    android:smallScreens="true"
    android:xlargeScreens="true" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme" >

    <!--
         The API key for Google Maps-based APIs is defined as a string resource.
         (See the file "res/values/google_maps_api.xml").
         Note that the API key is linked to the encryption key used to sign the APK.
         You need a different API key for each encryption key, including the release key that is used to
         sign the APK for publishing.
         You can define the keys for the debug and release targets in src/debug/ and src/release/. 
    -->
    <meta-data
        android:name="com.google.android.geo.API_KEY"
        android:value="@string/google_maps_key" />

    <activity
        android:name="com.ponny.radiomobile.MapsActivity"
        android:label="@string/title_activity_maps"
        android:configChanges="keyboardHidden|orientation|screenSize">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

    <meta-data
        android:name="com.google.android.gms.version"
        android:value="@integer/google_play_services_version" />
</application>