Made OnboardingFragment more customizable

Exposed moveToPreviousPage and moveToNextPage so subclasses
can define custom behavior e.g. auto-play vs. manual mode, and
call these methods directly.
Also created a listener to notify when views become visible.

Test: Used a subclass of this fragment within an app that can
work in both auto-play and manual mode. Tested for Talkback in both
N and O.

Change-Id: I739259e1ca249a58a766a8271a41c9dd3e3308e6
3 files changed