[web] Olm session creation between own devices

Summary:
ENG-7221 : Tunnelbroker session creation on web

Now that we have a proper olm session persistance on web we can implement testing screens for peer to peer message sending, and creating sessions between own devices.

getContentSigningKey and createOlmSessionsWithOwnDevices functions have been moved from native to lib and updated to use the cross-platform APIs.

Depends on D11340

Test Plan: Created sessions from web to native and tried sending messages both ways. Repeated with native creating the session to web. In both cases the messages where successfully ecnrypted/decrypted.

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11341