Fix scanning app data with metadata

Fix scanning app data with metadata

Previously we were listening to change in the appdata folder but an
appdata scan didn't setup the file system, so the view was unavailable.

**Test plan:**

1. rm -rf data/appdata_...../preview data/<user>/Media
2. occ files:scan-app-data
3. occ files:scan <user>

No errors and the files and metadata are correctly removed from the
database too.

Signed-off-by: Carl Schwan <carl@carlschwan.eu>