TabLayout + ViewPager improvements

- Add support for arbitrary decor views in ViewPager.
  Previously it had a hidden marker interface, but now
  we have a public class annotation.
- Made ViewPager's internal adapter change listener public
  to enable decor views to receive any changes.
- TabLayout now supports being a decor view by automatically
  setting itself if it has a ViewPager parent.

BUG: 27812405
Change-Id: Ia1a369f9401ee0108544a4aacc1461db3639cc80
6 files changed