Call pause() and resume() on child animators.

Fixes: 269529338

When AnimatorSet was paused or resumed, the child animators
were not being paused or resumed. This CL makes fixeds that.

This CL also updates the iteration over children so that
temporary allocations are reduced. A cached array of items
are saved and reused for the next iteration when possible.

Test: new tests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:218395a67afc6a16a53b5bef8d7c6fa764bed9b4)
Merged-In: I40fa2115099116cfe9320b0587948f08a8650f91
Change-Id: I40fa2115099116cfe9320b0587948f08a8650f91