android: Add an adaptive launcher icon

Using <inset> in a mipmap folder apparently is not fully valid, at least
Android Studio complains about it (it seems to work fine, though).

src/frontends/android/app/src/main/AndroidManifest.xml

@@ -24,7 +24,7 @@
 
     <application
         android:name=".logic.StrongSwanApplication"
-        android:icon="@drawable/ic_launcher"
+        android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
         android:theme="@style/ApplicationTheme"
         android:allowBackup="false" >