[lib] Update MinimallyEncoded*Info types to be $ReadOnly

Summary:
Updated the MinimallyEncoded*Info types to be $ReadOnly since spreading the "base" types drops the "read only" property and makes them writable... which caused some Flow issues.

Also going ahead and adding minimallyEncoded discriminator property to all of the types as we did for MinimallyEncodedRawThreadInfo in D9766 to help Flow refine types.

Test Plan: Flow, updated unit tests

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: tomek

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9788