Deprecate the term "submodel" in favor of "step model", "referenced model", etc

With the introduction of control flow and referenced models,
the term "submodel" becomes easy to misinterpret. This change replaces existing
uses of the term "submodel" with "step model" and "sliced model" depending on
the context.

Bug: 139181916
Test: NeuralNetworksTest_static_asan
Change-Id: Ida0dd90f983321f4aa3664763afa331d8d0da4ca
7 files changed