[lib] Introduce decodeMinimallyEncodedRawThreadInfo

Summary:
This allows us to "recover" RawThreadInfo from MinimallyEncodedRawThreadInfo encoded via minimallyEncodeRawThreadInfo.

This is *finally* the last utility function that we'll be introducing in this stack. We now have all of the building blocks in place and can make necessary changes to keyserver/native/web.

This function is also "temporary" because in the future MinimallyEncodedRawThreadInfo will effectively be the new RawThreadInfo. Usage of this function will be replaced by refactoring client code to query the minimally encoded permissions directly.

Depends on D9752

Test Plan: Unit test, will add more unit tests at end of stack with "prod data" from my Redux store.

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9755