Volume buttons should control media volume, not ringtone volume

Currently when you use your volume buttons it adjusts the ringtone volume of the phone, which has no effect on this app's volume. This shows how to make it affect media volume instead: https://github.com/ankidroid/Anki-Android/blob/dbbf3ea6549e82dee9e3c0df39bb5c3f74d3e189/AnkiDroid/src/main/java/com/ichi2/anki/AnkiActivity.java#L57

Using version 1.4.2.