title: How to change toolbar title with fragment back stack
body: <p>I have made a very simple application with multiple fragments in one activity. Also, added toolbar.</p>
  <p>Now 4 fragments have different titles.</p>
  <p>when I go to fragment 1 the title is <strong>Fragment 1</strong></p>
  <p>when I go to fragment 2 the title is <strong>Fragment 2</strong></p>
  <p>now come to the problem....</p>
  <p>when I press back from fragment 2,</p>
  <p>I came to fragment 1.</p>
  <p>but the title in the toolbar is still <strong>Fragment 2</strong></p>
  <p>This is the problem I want to solve.</p>
  <p>Please help me to fix this.</p>
  <p>Explain me in easy steps (prefer java)</p>
  <p>Thank you in advance.</p>