Migrate AuthDb to the Element X session storage

[bmarty]

In order to let the EA application to upgrade to EXA codebase, we need to first migrate the existing session credentials to the EXA session storage.

After a quick check, we need to also migrate the data on EXA to have two new field that are present to EA:

• isTokenValid: Boolean used to store the fact the the token is not valid anymore (401). EXA does not yet properly handle this user story
• loginType: String. Values are here

Also, EXA request to store the sliding sync url, this is not done by EA. This is quite a blocker here, since EA does not require a sliding sync proxy to run on. We may default to the homeserver url, or retrieve it again from the .well-known, but we cannot do network request during the database migration...

Context: element-hq/element-x-android#86