[skottie] Disable on Android Framework builds

Make Skottie truly optional (own flag), and disable in framework builds
(to unblock landing the RapidJson refactoring).

Change-Id: I4611f915e43fe11c1f6754ab4a9f63e45af2f8d3
Reviewed-on: https://skia-review.googlesource.com/125872
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Klein <mtklein@google.com>
7 files changed