[androidkit] expose saveLayer

Additionally, pass return value from native save and saveLayer to java

Change-Id: I1a4e04305ba8e57f6a639978b00f8e2ca1a59f90
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/415170
Commit-Queue: Jorge Betancourt <jmbetancourt@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
2 files changed