title: I want to clear the value of the shared preference
body: <p>I have stored the username and the password in the sharedpreference.
  And I am displaying the username in every activity like Welcome "Username".
  But at the time of logout I have put one checkbox in the dialog box.If the check box is checked the sharedpreference value should be clear. So I don't know how to do it.Please help me out of it. Thank you.</p>