Declare OCA.Search directly, not via a monkey patch

Apps could use OCA.Search very early on a page load without a problem
prior to Nextcloud 20 (current master). Since the unified search work
moved some code around, it now was added later, resulting in `TypeError:
OCA.Search is not a constructor` errors if the apps did not wrap their
calls in a `DOMLoaded` event handler.

To ensure existing code continues to work without modification, this
patch moves the declaration of the `OCA.Search` API to where we defined
`OCA`, instead of monkey-patching it later on.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>