Add CompositingDecisionAtAnimationPhaseBoundaries feature flag

Previously we forced an update of the compositing decision when an
animation enters or leaves the active phase in order for native paint
worklet (NPW) animated properties to work with view timelines. This
change introduced rendering glitches for view timeline based animations
of opacity. The decision whether to force a new compositing decision is
now behind a feature flag to restore the old behavior. This change
intentionally reintroduces a bug with NPW, but as they are still in
experimental, they can be safely fixed in a followup CL.

Bug: 403708813
Change-Id: I780ef8235d40346a75159980b44eff7d775222b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6361481
Commit-Queue: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434090}