Pinned animation: Avoid expensive defensive programming.

Sys-trace revealed contention here as the overwhelming
cause of missed frames during the animation (typically contention
with relayout). The checks aren't necessary, and there is a visible
gain from removing the contention here.

Bug: 35396882
Test: Manual
Change-Id: I5151c603ac445c7832c9c499b663e5b9ec54e9f4