Fix initialization of AnimatorSet

Bug: 289165914

When I previously changed the initialization code for AnimatorSet,
I had simplified a section because the initialization contained
all of the complexity. I later reverted the change to how I
initialized the child animators, but did not revert a part of
the code.

This reverts the change to initialization during start.

Test: broken animation sample, new test
Change-Id: I7333865f677f42964b023bcd9ee768ff4ec0137d