Is there a way to set drawable’s Alpha using XML?

It’s been a while since the OP, but personally found a solution that worked a lot better for me than the suggested answers. Creating a BitmapDrawable makes is easily possible to set the alpha:

<?xml version="1.0" encoding="utf-8"?>
<bitmap xmlns:android="http://schemas.android.com/apk/res/android"
    android:src="https://stackoverflow.com/questions/8179250/@drawable/your_drawble"
    android:alpha="77">
</bitmap>

Alpha can be any value between 0 and 255. Note that it is sort of the inverse of the HEX color value alpha, as for example 70% alpha would be B3 in HEX and 77 in the BitmapDrawable.