Which mobile SDK class/classes contains the method for obtaining the OAuth access token?
Token issuance is Mobile SDK's internal process and was never meant to be exposed to the application developer.
The Internal process is that Mobile SDK takes extended MASAuthCredentials class based on grant_type, and performs registration and authentication as per the grant_type.
The whole purpose of MASAuthCredentials is to provide flexibility to developers to override parameters, and headers of these registration and authentication requests.
When the extended MASAuthCredentials was passed into Mobile SDK, the Mobile SDK makes the request for registration and authentication with the given parameters and headers from the extended class.