[lib] Rename RawThreadInfos to MixedRawThreadInfos

Summary:
Part of https://linear.app/comm/issue/ENG-6366/rename-minimallyencodedrawthreadinfo-to-rawthreadinfo

New type looks like:

export type MixedRawThreadInfos = {
  +[id: string]: LegacyRawThreadInfo | RawThreadInfo,
};

which is slightly different than

LegacyRawThreadInfos | MinimallyEncodedRawThreadInfos because it allows values of both types.

However, we should be able to now replace most instances of MixedRawThreadInfos with LegacyRawThreadInfos | MinimallyEncodedRawThreadInfos. I think we only really will need MixedRawThreadInfos when we're working with SQLite store that may contain either (eg in case we're upgrading from an old version and applying migrations before encoding change).

Depends on D10634

Test Plan: flow

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10637