Return iterators for iterating over animation and keyframe properties.

This avoids the cost of constructing a PropertyHandleSet, while still
allowing directly iterating over the return value.
KeyframeEffectModelBase::Properties() will iterate over the same
property for each keyframe it exists in, callers should locally
construct a PropertyHandleSet if they require uniqueness.

Bug: 401059729
Change-Id: If495f46fa5f11dd8a053c519a64a1f5dff0e9dd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6356083
Commit-Queue: Robert Flack <flackr@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1441157}