Return correct year in DayPickerPagerAdapter.getYearForPosition()

Previously it wasn't offsetting the position by the min date's month,
so it would be off by a year if the min date wasn't in January. Also
now updates button visibility whenever the ViewPager's range changes.

Bug: 21203303
Change-Id: Id7a41a4fa48843943c744fb0fecd4bebb798cee5