Control the size of a modal dialog layer in android

Is it possible to control the area where the modal dialog is blocked in android? I have a custom dialog implementation that extends ProgressDialog. It locks the screen when the dialog is visible and can be canceled by pressing the back button. I have a navigation at the bottom of the screen and I want the dialog not to block them, i.e. The rest of the screen has been locked and only navigation is still clickable so that if the user decides during the loading screen that he wants to leave, he may not have to wait for him to finish loading the screen he is not interested in. Can anyone point me in the right direction on how this can be achieved?