ref(android-nav3): Rename observer helper classes

Rename the local BackStackObserver collaborators to a consistent Nav* pattern.

These types exist only to simplify the observer's implementation rather than to model stable domain abstractions. Using a shared helper-oriented naming scheme makes that status clearer and keeps the file's private implementation pieces on the same footing.

Co-Authored-By: OpenAI GPT-5.4 <noreply@openai.com>

Author: 0xadam-brown