manifest to report correct favicon size

In the environment where php-imagick with SVG support is correctly installed, this endpoint returns 512x512 image.
https://github.com/nextcloud/server/blob/d9015a8c94bfd71fe484618a06d276701d3bf9ff/apps/theming/lib/Controller/IconController.php#L146



The problem is in the environment without php-imagick or without SVG support, this will fall back to original, whose default is 128x128; but I thought this is a separate problem. (For this, default image may be enlarged. Should I create another issue if this is important one?)

Signed-off-by: nhirokinet <nhirokinet@nhiroki.net>