Fix spring not springing when unstashing.

The spring animation was getting cancelled by the reset
animation.

Added a check to avoid creating a new reset animation if we
are already animating to the final value.

Bug: 273961611
Change-Id: I3afb62b89b5f6fbe920906499db2497ef8e94069
Flag: ENABLE_TRANSIENT_TASKBAR
Test: stash transient taskbar