Refactor AnimatorSet in prep for adding more functionalities

This refactor changes how relationships (i.e. with/after/before)
among Animators in an AnimatorSet are represented. Specifically,
a directed graph is used: parent-child indicates sequential
relationship, and siblings will fire simultaneously.

This CL also fixed the issue where start delays for Animators that
play together are not handled correctly.

Bug: 11649073
Bug: 18069038
Change-Id: I5dad4889fbe81440e66bf98601e8a247d3aedb2e