This reduces the acceptable forms for hook handlers to three things:
* a callable (in the form of a string, an array, or a closure)
* an object, which is expected to have a public "on" method that
matches the hook name.
* an array containing an object spec in the "handler" key, for use
with ExtensionRegistry.
All other forms will trigger a deprecation warning.
Bug: T339167
Depends-On: I980f2d45e6bb8c6a04058e68c758f71bbcf709de
Depends-On: Ieae405f70caa01d84602583cc214b0ee3fadc796
Depends-On: If15df4b598c02ed9bda5eea0ae89a16ebbf4f2e2
Depends-On: Id70276fa1e1821bd400dc0ae5cea722a21d524d5
Change-Id: I83bc81d1b3033c38b9313884a9c70a187fdde227
c4033734db