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
Merged-In: Ida0dd90f983321f4aa3664763afa331d8d0da4ca
(cherry picked from commit d9baa152f149d91a95bab802bec10c97c62e0592)
7 files changed