[skottie] Add onTextProperty support into PropertyObserver.

This effectively allows observing/modifying properties at the text document level. By default
Bodymovin exports even static TextDocuments as a single keyframe, so the unit-tests have been
updated to store the property handles for verification.

Change-Id: Iab8bcb29cdc5626d1abc34593ee9967b543428eb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/231681
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
6 files changed