Reuse a base class [Activity] handler in all sub classes for background work
You make some assumptions about your window in this line:


mHandler = getWindow().getDecorView().getHandler();



As mentioned by the Activity docs for getWindow()


  Returns
  Window The current window, or null if the activity is not visual


Okay. NullPointerException number 1 waiting to happen. Interestingly #getDecorView() seems to be guaranteed to not return null, which is good. One place less where this can fail.

It is irrelevant whether the returned handler is null or not, because luckily that's what this is about.



Other nitpicks:


Don't use Hungarian notation. mHandler should be just handler. Your IDE is perfectly capable of telling you that handler is a member of type Handler
Failing to reuse an activity handler is not something I'd categorize as Warning. 
The comment you place there is partly restating the obvious. Drop the second sentence.