[androidkit] Start implementing reusable samples

Introduce a Sample interface for abstracting out various demos/samples.

Implement a simple image shader sample for starters.

Update the cube demo to map samples to sides.

Change-Id: I9c9c70fa05634404ced7bd30739f6e884546b91d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413441
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
4 files changed