clean playback methods and constructors in SkottieView

playback methods would call on mAnimation before checking if it was null
builder constructor created unneeded confusion, as setSource() currently acts a build method

Change-Id: I90a36e39cab1ed76a12c49fb53b4518bba9bf4ef
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/340521
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
3 files changed