Legacy Parts: Update PagerSlidingTabStrip class

-- Check if tabCount > 0
(do this before attempting tabsContainer.getChildAt(position) when scrolling to child.)

-- if pager == null no need to call requestLayout()

-- make shouldExpand work correctly
   (referenced here: https://github.com/astuetz/PagerSlidingTabStrip/pull/42)
-- Use Gravity to center tabs

-- add onTabReselected() callbackFunction
  ( referenced here: https://github.com/astuetz/PagerSlidingTabStrip/pull/92)

Change-Id: I0184de3c010a902a721572d3dcaa75101e38aa0c
Signed-off-by: Andre Saddler <dresadd09691@gmail.com>