[PB6] Encapsulate the buttons into a view

Handling buttons directly in the preference causes large
problems of code readability and interface.  It's better to
have a class to manage the buttons and their animations
separately. This is feature-equivalent, and mostly
delegates stuff for now.

Bug: 7600384
Change-Id: Ia8da0ec68ffac84fc1d65e1760539a87a73fa776