[crypto] make sure short format of prekey is expected when creating an outbound session

Summary:
on native, we were still uploading the long prekey format to the identity service. additionally, web and native, we were still expecting the long format when trying to create an outbound session.

changes in this diff:

• expect short format on native/web when creating an outbound session
• dedup prekeyBlob parsing
• add prekey parsing to getOlmSessionInitializationData

Depends on D11312

Test Plan:

1. created notif session on native
2. created notif session on web
3. created content session on native

Reviewers: marcin, kamil

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11319