Improve PagerSnapHelper flinging and documentation.

- Expose createSnapScroller method in SnapHelper to allow extending
  classes to change snapping scroll speed.
- Limit the maximum duration that the scroll animation takes on flings.
  This limit is now 398ms:
  - 100ms for scrolling
  - 298ms deceleration (100ms /.3356 <- see LinearSmoothScroller)
- Added more documentation for PagerSnapHelper class.

Test: passes the existing tests.
Change-Id: I5d0244f44aefc91b82ee4b4145e453da3c9112e6
4 files changed