Doesn't access database directly anymore but goes through AxolotlService
now to obtain list of fingerprints associated with an Account/Contact.
This should prevent orphaned keys littering the UI which previously
couldn't be removed through the Clear Devices function.
Together with 1c79982da84964c1d81179a0927d9cd1eadf53de this fixes#1393
XmppAxolotlMessage is now entirely responsible for handling encryption
and decryption of messages, only leveraging XmppAxolotlSession as a
packing/unpacking primitive for payload keys.
Removed pseudo-dead session generation code step from prepareMessage
function, as sessions have been created by invoking the
TrustKeysActivity for a while now.
Added prepareKeyTransportMessage function, which creates a message with
no payload. The key that is packed into the header keyElements can then
be used for other purposes (e.g. encrypted file transfer).
Moves SQLiteAxolotlStore and XmppAxolotlSession into proper classes.
IdentityKeys trust statuses are now cached in an LruCache to prevent
hammering the database when rendering the UI.
