Add basic support for input shaders to RuntimeShader.

This adds the ability for Java RuntimeShaders to provide other
shaders as inputs.  This is a first pass API and should be updated
to mirror the SkRuntimeShaderBuilder API that enable callers to set
the uniforms and inputs shaders by name instead of passing arrays.

Test: HwAccelerationTests
Bug: n/a
Change-Id: I53a15ad864bede2fecc1e2459dca983d224114a0