[iOS] Separate command handlers in OverflowMenuMediator

Best practice is not to bundle command handlers together into a single
object, and not to name it "dispatcher". The overflow menu mediator was
also inconsistent in that some handlers were bundled and others weren't.

This CL switches to use specific per-protocol handlers in the overflow
menu mediator. This will also make it easier to decouple the settings
commands from ApplicationCommands.

Bug: 906662
Change-Id: I2f100fc9ed076dcc010ef3ab1bc8b42d7e905ae1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4941919
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Cr-Commit-Position: refs/heads/main@{#1210867}