Fix fastscroller not resetting when switching tabs.

Whenever fastscroller is binded, reset the scroll position so its at the top.

bug: 335520751
Test video manually:
before:https://drive.google.com/file/d/1MOE0LOL8G5pmhJYf61y9jw2nDVXIvaAs/view?usp=sharing
after: https://drive.google.com/file/d/14Yi6JRMvDhDZKNwusv75DtfeGk1A4W23/view?usp=sharing
Flag: NONE bugfix

Change-Id: I7aea9a7fe5582fbb61e979f375191980c8a7e0ad