[androidkit] refactor Surface.getCanvas to be implemented at a subclass level

Change-Id: If726f4fc921ba84dd5498ab9f99fbd1a747398b1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/403676
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
3 files changed