OLMKit: Add wrappers for export/import of inbound group sessions

Merged from https://github.com/aapierce0/OLMKit/commits/macOS_port

Sign-off taken from https://github.com/matrix-org/matrix-ios-sdk/pull/218#issue-201062079.

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.