feat(aio): implement a way to check and update a PR\'s preview visibility

Previously, `BuildCreator#changePrVisibility()` would throw an error if the PR's
visibility was already up-to-date or if the PR directory did not exist (e.g. was
removed). This method was only used from inside `BuildCreator#create()`, which
had already checked for the existence of the directories.

This commit renames `changePrVisibility()` to `updatePrVisibility()` and makes
it more "forgiving" (i.e. it will only throw if both public and non-public
directories exist). This allows it to be used on events that may or may not have
caused the PR's visibility to change (e.g. a GitHub webhook triggered whenever a
PR's labels change).

aio/aio-builds-setup/docs/overview--http-status-codes.md

@@ -42,10 +42,6 @@ with a bried explanation of what they mean:
 - **403 (Forbidden)**:
   Unable to verify build (e.g. invalid JWT token, or unable to talk to 3rd-party APIs, etc).
 
-- **404 (Not Found)**:
-  Tried to change PR visibility but the source directory did not exist.
-  (Currently, this can only happen as a rare race condition during build deployment.)
-
 - **405 (Method Not Allowed)**:
   Request method other than POST.