Avoid instantiating ExtensionMojoBinderRegistry on read-only queries

Previously, ExtensionMojoBinderRegistryFactory::GetForBrowserContext()
always created the KeyedService instance (`create = true`). This caused
empty registry instances to be instantiated on demand in profiles where
no feature registered an ExtensionMojoBinderProvider (e.g. Guest or OTR
profiles) whenever Mojo JS enablement or interface binders were checked.

This CL updates `GetForBrowserContext()` to accept a bool and updates
the callsites to pass `/*create=*/` and null-check the registry
appropriately to prevent unnecessary KeyedService creation and avoid
ProfileKeyedServiceBrowserTest failures.

Bug: b:524201650
Change-Id: I7d7d9f6b4bc03848436581c5cbf1f58d40a3e7f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8277313
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Andrea Orru <andreaorru@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1684462}