Also fixed the issue of missing onPageSelected callbacks upon first entry;

The logic for automatic updates keep same with Android, as follows
1. If the previous item only changes its location,
update the current location and keep the current item displayed.
2. If the previous item does not exist, do not adjust the position,
but keep the current position in the valid range (that is, 0 ~ count-1).

this is a minor fix for feat 'pageIndex of ViewPager auto update after data changes'