restore fab icon when going back

Signed-off-by: parneet-guraya <gurayaparneet@gmail.com>

app/src/main/java/com/owncloud/android/ui/activity/FileDisplayActivity.java

@@ -1025,6 +1025,7 @@ public class FileDisplayActivity extends FileActivity implements FileFragment.Co
      */
     private void popBack() {
         // pop back fragment
+        binding.fabMain.setImageResource(R.drawable.ic_plus);
         resetScrolling(true);
         popSortListGroupVisibility();
         super.onBackPressed();