title: Android Marshmallow Callback when user revokes permissions from settings ?
body: <p>Is there a way to get callbacks if the user revokes permissions from the settings after having granted them? </p>
  
  <p>I know we have to check for permissions before trying to use a camera etc etc. </p>
  
  <p>Consider the following scenario, I have a video processing application which needs permission to read and write to storage. The task of processing is usually in the order of seconds. Assuming the user gave permissions when asked and revokes them after the processing starts. 
  In this case is there a way we can get callbacks rather than we checking each time? Just so that we can handle the error case gracefully.  </p>