When an app (e.g. richdocuments) calls registerHandler() from the
@nextcloud/viewer package, the handler is stored in the global
window._oca_viewer_handlers Map. These handlers are only bridged into
the ViewerService (and thus into OCA.Viewer.mimetypes) by a
DOMContentLoaded listener in Viewer.vue.

Since all Nextcloud scripts are loaded with the defer attribute, there
is a race window between the file list rendering and DOMContentLoaded
firing. If the files app finishes rendering before the bridge runs,
viewerAction's enabled() check reads OCA.Viewer.mimetypes before
document mimetypes have been added — the viewer action is disabled
and the download action takes over, causing documents to be downloaded
instead of opened in Collabora/the viewer.

Fix this by also registering the handler directly with the
ViewerService when it is already initialized. Because viewer-init
runs before other apps' init scripts (both are deferred, viewer
registers its LoadViewer listener first), OCA.Viewer is guaranteed to
exist when richdocuments' init-viewer script calls registerHandler().
Duplicate registration is harmless — ViewerService.registerHandler()
already rejects handlers whose id is already known.

Signed-off-by: Andras Timar <andras.timar@collabora.com>