Removed possibility to generate weak RSA keys (512 and 1024 bit)

This is reference to issue https://github.com/openpgp-keychain/openpgp-keychain/issues/102. I disabled possibility to pick RSA 512-bit and RSA 1024-bit keys from CreateKeyDialogFragment - they may be selected from Spinner, but if so, "ok" button will be disabled. In second commit I added note about that (for user).

I'm unsure what "editing existing keys with 512, 1024 rsa" exactly mean - I've searched such option in app and in code - and I haven't found any. This issue is 6 months old, so maybe meanwhile this possibility was removed? Please let me know how this should be done properly if this solution is wrong.