[lib] Extract handling Olm P2P messages to a separate function

Summary:
Code for handling ENCRYPTED_MESSAGE is expected to grow really large, so extracted this logic to a separate hook.
It can be split further into smaller functions when it grows too large (e.g. separate function for user actions).

Depends on D12636

Test Plan: Flow. Also checked secondary device logout - it still works as expected.

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12637