Minor style things
Introduce Key/KeyRing wrappers

The goal of these classes is to greatly reduce the needed static helper
classes that work on BouncyCastle key ring and key objects.
Ideally only the pgp package uses BouncyCastle code, the rest of the
app should use this abstraction.

PublicKeyRing and SecretKeyRing exist only for cases where it is vital
that a specific kind of key ring is needed, for instance when it comes
to uploading to a key server, where a secret key ring must never be
used by accident.
Use hi-res version of the icon
Fix Key and KeyRing constructors
Fix expiry date picker dialog
Adjust theme and icons
Make PgpSignEncrypt use Key/KeyRing and PgpKeyProvider

This divorces the code from the app's content provider and removes
the need for PgpConversionHelper in parts of the code already.
Change PgpDecryptVerify to using PgpKeyProvider as well
Make Key serializable and use it for key editing

Also further reduce the usage of PgpKeyHelper and PgpConversionHelper.
Move key ring armoring into KeyRing, make code use it

Also remove PgpConversionHelper, as it isn't needed anymore.