pause/resume for Animators

It is now possible to pause Animator-based animations. Pausing an
animator causes it to hold the current time/value indefinitely, or
until end/cancel/resume is called. When resume() is called, it continues
from where it left off.

There is a new listener interface on Animator, AnimatorPauseListener,
which can be used to listen to pause/resume events.

Change-Id: I77d1535e792fb7bf349f549a0ac0a0d85958cb47