android: Add activity to import VPN profiles from JSON-encoded files

The file format is documented on the wiki.

URLs to .sswan files may be intercepted and downloaded files with a media
type of application/vnd.strongswan.profile may also be opened (the file
extension doesn't matter in that case).  Whether downloaded files for which
the media type is not correct but the extension is .sswan can be opened
depends on the app that issues the Intent.  For instance, from the default
Downloads app it won't work due to the content:// URLs that do not contain
the file name but when opening the downloaded file from within Chrome's
Downloads view it works as these Intents use file:// URLs, which contain
the complete file name (the latter requires a new permission).

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