[androidkit] Introduce RuntimeShader

 - opaque Shader class to act as a Java SkShader wrapper
 - shader slot on Paint
 - RuntimeShaderBuilder utility to enable the same use pattern as native
   Skia:

  RuntimeShaderBuilder builder(sksl_string);
  builder.setUniform("foo", 1);
  builder.setUniform("bar", 2);
  paint.setShader(builder.makeShader());

or, more fluent:

  paint.setShader(
      RuntimeShaderBuilder(sksl_string)
          .setUniform("foo", 1)
          .setUniform("bar", 2)
          .makeShader());


Change-Id: I7cd241a2f64bc54dcae2175ed35040edf6506ed3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/399736
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Mike Reed <reed@google.com>
11 files changed