Explicitly set android:extractNativeLibs to true in ApplicationManifest.xml

The Android Gradle Plugin's default for android:extractNativeLibs changed from
true to false beginning with version 3.6.0. Based on GeckoView's needs, we
should ensure that this attribute is explicitly set to true.

app/src/main/AndroidManifest.xml

@@ -25,6 +25,7 @@
     <application
         android:name=".FenixApplication"
         android:allowBackup="false"
+        android:extractNativeLibs="true"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:roundIcon="@mipmap/ic_launcher_round"