Android's scheme matcher is case-sensitive, add 'openpgp4fpr' variations

Just a few guesses at the most likely variations of case that someone might
think up.  Also, it turns out that QRCodes have a special all UPPERCASE
mode which makes the QR Code have less data, and therefore larger blocks
which are easier to scan.

refs #1594
handle Exception when getKeyByFingerprint() does not find a key

If you give GnuPG.context.getKeyByFingerprint() a fingerprint to a key that
does not exist in the keyring, it throws an Exception.  Treat this
Exception as a message that there is no key with that fingerprint in the
keyring.
if EXTRA_TEXT is <200 and has a fingerprint, launch ReceiveKeyActivity

This lets you roughly select the text with a PGP fingerprint in it, send it
to GPG, then have GPG --recv-key from the keyservers.