Add ViewPager2 support

[angusholder]

Add DotsIndicator.attachViewPager2(ViewPager2) method to allow dots indicator to be used with ViewPager2 as well.

I added viewpager2 as a compileOnly dependency, so people who don't use attachViewPager2 won't have ViewPager2 pulled into their project.

assets/sample.apk needs recompiling, did you just do an unsigned debug build to create that?

Fixes #2