Preserve tab position

To reproduce the issue:
1. Swipe to work tab
2. Tap Home
3. Swipe up to show all apps, you can see the personal tab is
   highlighted while the view pager is showing work apps.

We cleared the state whenever onLayout is called, we should
make use of mSelectedPosition.

Change-Id: I621b3c9ada0eb608280fe5e4ea8715f3a341820d