registration: Allow string value for Hooks

registration: Allow string value for Hooks

Instead of forcing an object even for single-hook-listeners, allow
string values, too (one hook listener for one hook, only). Also:
use it as default for the conversion script, if only one listener
is added to a hook (which is usually the case). This leads into a
much cleaner output of the Hooks section of extension.json.

Bug: T133628
Change-Id: Ie9e54f0931c41706eb8d82d00256698992ec41cc