Add hooks to 'installed' lifecycle callback for overwriting existing registration

[Alex Wei]

An app should be allowed to veto overwriting existing registration with the same client id, for example, it must unregister first by calling 'uninstalled'.

An app should be allowed to veto overwriting existing registration with different client id but the same base URL.

An app should be notified of such overwriting so as to do proper clean up and setup.