wrap bouncycastle objects and don't use them in android-related code

What's in here:
* wrapper classes for all relevant bouncycastle objects
* no more bouncycastle imports outside the .pgp packages
* all "getter" methods from ProviderHelper moved into CachedPublicKeyRing, which extends KeyRing so provides the same interface as the wrapped keyring objects 
* new SaveKeyringParcel. this is a WIP pending the new Edit Key activity

Stuff that I know is broken at this point:
* Key creation is broken -> because of pending rewrites in key editing code
* Fingerprints are not checked -> because of pending rewrites in the key import code

Making a pull request here because it's quite the handful so I better not commit to master. But stuff is piling up, so if someone feels like reviewing and/or commenting... :)