Export and import of megolm session data

Android wrappers for olm library

-> Update the deserialization methods (the jni methods create an object instance before deserializing them.

olm_import_inbound_group_session, which reads the format written by
olm_export_inbound_group_session to initialise a group session.

A pair of functions which allow you to export the megolm keys for an inbound
group session, so that an application can save/restore them.

-> remove javaCStringToUtf8

-> Always use javaCStringToUtf8 to convert byte array to String