[lensoverlay] Clean up style guide violations with "ForTesting" methods.

Includes the following changes:
- Converts the name of simple accessors to snake_case().
- Un-inlines the rest.
- Moves the declaration and definition of `GetTabInterface()` so it's not sandwiched between a bunch of testing methods.

Change-Id: I5c36ffc9a0e7dea41c0b8d11564ec5f297caa08f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5619011


Reviewed-by: Erik Chen <erikchen@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313152}