Avoid double invocation in PipExpandAnimatorTest

Intentionally do not start the PipExpandAnimatorTest when testing the
onAnimationUpdate, to avoid double invocation and focus on the update
listener itself.

Flag: com.android.wm.shell.enable_pip2
Bug: 417356890
Test: atest --iteration 20 PipExpandAnimatorTest#onAnimationUpdate_expand_setRoundCornersWithoutShadow
Change-Id: I6af03f01a99abd2e0389164ef1a87e9572b1755f