[animation-trigger] Disable InertEffect if a trigger is present

When an Animation has an AnimationTrigger, it will not play until its
trigger tells it to. If this Animation's fill mode is such that it
should only have an effect in the active phase, we should prevent its
InertEffect from having an effect before the animation enters the
active phase.

This patch achieves this by leaving the InertEffect's local time
unresolved until the trigger plays it.

Bug: TBD
Change-Id: I7d4dc141780ccf6633b5e25dd26ba7e359fba58b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6391419
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: David Awogbemila <awogbemila@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1438829}