Don't allow the caller to distinguish different failure modes

[mgiuca]

Raised by @dbaron on the TAG review.

There is a minor privacy issue in distinguishing between "no apps are available" and "the user cancelled the share"; if only a handful of popular targets exist for a particular payload (especially if we add new share types that might have specialized handlers, e.g., for rare MIME types) then a malicious site could use navigator.share to check whether those apps are installed, based on the response string.

Also, in our current Android implementation, we are totally unable to distinguish between "no apps are available" and "the user cancelled the share". Therefore, let's just remove this provision. I won't go as far as to explicitly bar the user agent from distinguishing these, but add a note under security considerations.