AnimatorSet sends pause/resume for seeked animators

Fixes: 270709315

This CL makes it so animators that are part of an
AnimatorSet that has been seeked will send
onAnimationPause() and onAnimationResume() when
the AnimatorSet is paused and resumed.

Also made AnimatorSetActivityTest#testIsRunning()
more robust by checking state on the UI thread.

Test: new test and ran previous animator tests
Change-Id: I20fed31c626f9bf1d665169a61a37ea1307d4790