Improve AnimatorSet seekability.

Bug: 259287604

Improved seeking for AnimatorSets so that they animate their
children forward and backwards, in the proper order. Also,
seeking in reverse is now allowed, while the AnimatorSet
is paused.

Test: new CTS tests
Change-Id: I83ac14465be1666496b06db97ac0067ffc6529e6
3 files changed