title: modify the transparency of a background image in android xml
body: <p>Hi i need to put a background image in an activity and this image needs to be a bit transparent because i want to put text over it. how can this be done?
  can it be done directly on the xml file and if yes how? 
  Is it also possible to do it programmatically and if yes how?
  And could you please show me an example.
  Thankyou!</p>