Don't load text file preview when text app is available (Fixes: #20615)

When opening a shared text document, the files_sharing app uses
internal preview support in case no other app to open the text file is
available.

So far, it only checks for the old files_texteditor app, not for the new
new text app. This commit fixes this by checking for both.

Signed-off-by: Jonas Meurer <jonas@freesources.org>